Disclaimer: Harry Potter is JK Rowling's. I make no money out of this.
Sirius' earliest memory was of the sound of shattering glass. He wasn't sure how old he was then, but he did remember his knees trembling (whether in fear or because he was still learning to walk, he never bothered to know). He had bumped into one of the end-tables in the narrow hallway of Grimmauld Place, and the next thing he knew, shards of broken (probably ancient) glass were all over the (also probably ancient) carpet. He was alone, and the bare end-table loomed threateningly over him. Not knowing what to do, he did the only thing a child his age did best: he scrunched his face up, mouth open wide, ready to cry and - the shards had gone, along with the carpet. Then a House-elf popped out of thin air if front of him, coaxing a surprised laugh out of little Sirius' still-open mouth, and the vase and carpet were forgotten - until that evening, when Walburga asked the House-elves about the missing decors. Nobody had even known they were missing, much less where they had gone. Little Sirius made no indication he understood what was happening. The Case of the Missing Vase and Carpet remained a mystery, and Sirius never gave enough clues to let anyone solve it.
When Sirius was 3, he accidentally dropped his baby brother. Regulus, just a little over a year younger than Sirius, wasn't supposed to be carried by Sirius in the first place. But let it be known that the eldest Black scion, even at age 3, never had a healthy respect for rules. Regulus, with the help of some illegally-obtained sweets, had been persuaded (by Sirius) to play piggyback (with Sirius). Regulus was a Dark Lord, Sirius a Dragon, and they were halfway up the mountain when Regulus' hands suddenly slackened around Sirius' neck. Sirius panicked and tried to re-secure Regulus' hold, instead losing his balance in the process. In a flurry of hands and feet, he instinctively took hold of the bannister. Regulus wasn't so lucky and had fallen all the way down. To Regulus' credit, he didn't cry - he merely sniffed and said Dark Lords don't cry. Sirius laughed and patted Regulus' back as he gave him more sweets. There was no blood, but the incident had resulted in Regulus' slightly asymmetrical skull. Fortunately, he was too young to remember, so Sirius didn't get in trouble. Just to be sure, he never told anyone anyway.
When Sirius was 6, he saw his mother fighting a stranger. He was playing Hide-and-Seek with Regulus and Kreacher, and he decided to hide inside an air vent in the Library. He had just replaced the metal cover back shut when his mother entered the Library with an unfamiliar man. Sirius could only see the gentleman's back, but it was clear that his mother was pulling at the stranger's hair, occasionally clawing at his back with her long, painted nails (Green, Sirius remembered clearly, because he noted during breakfast that morning that his mother looked like she had plants growing out of her nails). It was a strange sight, because his mother always used Magic whenever possible, and it made no sense to him now that she'd prefer Muggle combat over Spells. Still, Sirius kept his silence, because butting in on someone else's business was rude. When the adults used the Library fireplace to floo somewhere else, Sirius crawled out of the vent and dusted himself off. He thought about telling his father about what he had seen, but quickly decided against it - after all, Walburga fighting with someone wasn't exactly news. (Years later, he'd learn that that man was his father's business associate, and he most certainly hadn't been fighting with Walburga at the time. Even then, Sirius would keep his silence, just as he had when he was still hiding in that tiny metal space.)
When Sirius was 9, he first saw an un-moving picture. It was Regulus' birthday, and their parents had taken them to Knockturn Alley to let Regulus choose presents for himself. Sirius never liked the dingy alley, so he instead made up some excuse about being hungry and wanting to buy some food. He went to Diagon alone, just looking around, when he passed by a poorly-dressed wizard beside a magazine stand. He wasn't going to pay him some mind, but something strange caught his eye. At first, he didn't know what it was. Sirius stared at the stand - at the covers - waiting, until it hit him that the pictures weren't moving. The man said they were special non-Magic magazines, with a special spell cast specifically to make the images stop moving. Fascinated, Sirius picked five different ones (without checking the inside) and paid for them with the galleon his father had given him. Sirius didn't ask for the change and the man at the stand, eyes wide , bowed and thanked him profusely. Sirius returned to where he had left his family before, fingers crossed behind his back as he wished none of them would notice the extra padding beneath his coat. He never told his family that they had indirectly paid for his first Muggle items.
When Sirius was 11, he wasn't Sorted into Slytherin. He froze on the spot, staring wide-eyed at the underside of the tatty, old thinking Hat (asking what in Merlin's holy name was it thinking), as the last echoes of GRYFFINDOR-dor-dor reverberated in the silent Great Hall. With shaking hands, he lifted the blasted Hat off his head and returned it on the stool. As expected, the Slytherin Table looked out-of-their-depth, the Gryffindor Table even more so. No Black had ever been Sorted out of Slytherin before, not in centuries, not ever, until Sirius Black the Third. He was convinced his mother would kill him at the soonest opportunity (shame of my flesh!), but Sirius also knew that for now, he was out of her clutches, and he was free to do whatever the hell he wanted. So Sirius composed himself, stood straight (chin up, like he was taught), and said with what he hoped was a confident voice: So. That was serious, huh? In the impending nervous laughter (none from Slytherin), he smiled and took a seat at the Gryffindor table. Sirius never told anyone that half of him actually meant that as a real question and had expected for an answer.
When Sirius was 12, he learned Remus was a werewolf. He went straight to bed that night. He cannot join James and Peter in comforting Remus. He cannot even look at Remus' eyes! Werewolf! He was friends with a werewolf. How many full-moons had it been since First Year? How many instances had their lives been in danger? Werewolf! The screeching voice in his head sounded like his mother, and if only he could, he would silence it. It doesn't matter, he kept telling himself, but his voice sounded small compared to Walburga's. Shame! Shame on him for being weak. Shame on him for thinking he had changed for the better, only to revert back to what his family had shaped him to be when he was needed to be at his best. He tossed and turned, every movement punctuated by variations of Moony's innocence, how it wasn't his fault he was what he was. Sirius didn't get any sleep that night, but the next morning, he could look at Remus again. For then, that was all that mattered. After breakfast, James told Sirius about his plan to learn the Animagus Transformation, and Sirius replied with all the necessary encouragement. If any of them noticed that Sirius' eyes were bloodshot, they didn't show it, and Sirius never volunteered the information.
When Sirius was 13, he fell in love. He had sneaked off to a Muggle district with James during Hogsmeade visit and was passing by a junkyard when his eyes had fallen upon a beautiful vintage (You mean old and rusty, James had said, but Sirius just shushed him) Muggle motorbike. It took him two galleons and a rare Chocolate Frog Card, but he managed to convince James to help him bring the enormous contraption back to Hogsmeade. Sirius refused to shrink it, because the original mechanism might get damaged, and so they had to summon it in short bursts and distances all the way back (Sirius rode the bike since it would be suspiscious if it started moving without a driver) and James (hidden beneath his Invisibility Cloak) complained about underpaid workers for the whole duration of the trip. They hid it in the Shrieking Shack, and every Saturday night (minus full-moons) Sirius would come to fix it until the early morning hours. He liked the feeling of building something of his own, in private, without his parents' help or watchful gaze. He completed the repairs in six months, but he never told anyone so he could keep his peace his own.
When Sirius was 14, he fell off his broom during a match against Slytherin. Many were surprised, since this was Sirius, and Sirius was Gryffindor's star Chaser - he wouldn't just fall off his broom. But it had been raining hard, it was difficult to see clearly, and the fight was close; 170-150, with Gryffindor in the lead, and the snitch still loose. Each team's Seekers - James for Gryffindor, Regulus for Slytherin - were both more than a hundred feet above, looking for the elusive golden snitch in the hopeless deluge. The quaffle was (thank Merlin) with Sirius, in Gryffindor's possession, but the other Gryffindor Chasers (definitely not thanking Merlin this time) couldn't shake off their guards for more than a few seconds at a time. Left with no choice and only the Beaters' distant protection, Sirius flew as fast as his broom could carry him, whooping as he went past the center line, still uninjured and unstoppable - until he wasn't. James had swan-dived, Regulus close behind, and every pair of eyes in the Pitch followed both Seekers, including Sirius'. That was why when a Bludger came zooming towards both Seekers, Sirius - the nearest player - had time to rush in and block it with his shoulder. He woke up the next morning with his friends at his bedside and news of victory on their lips. James thanked him for the Bludger save, and Sirius smiled and waved like it was nothing. He never told James that he wasn't the only one he was trying to protect.
When Sirius was 16, he was disowned. It was a long time coming, and the only surprise was that it took that long for Walburga to blast his face off the Family tapestry. Sirius wandered aimlessly around London (thinking, not thinking, repeat), and when he came to, he was in Godric's Hollow, in front of the Potters'. When James saw him - in Muggle clothes, with a rucksack slung on his shoulder and a large blooming bruise on his cheek - there didn't need to be words exchanged. Sirius was thankful - he had had enough of words that day (cold eyes, cold words: You're no brother of mine). After a hot bath and a fresh change of clothes, he felt like himself again. James asked him if it hurt, and Sirius snorted, poking the bruise below his left eye (Have you seen the shit ton of rings on that woman's fingers? Of course it bloody hurt). He never told anyone that Regulus' words hurt more.
When Sirius was 17, James and Lily started dating. A few weeks before graduation, they were engaged. James had gathered the Marauders in the common room, grin ear-to-ear and Lily's hand in his as he told them the news. Sirius laughed heartily, congratulated both of them, and brought out the hidden bottle of smuggled Firewhiskey in his trunk. By the time the bottle was empty, Remus was already sleeping, Peter was dancing, James was smiling at Lily, Lily at James, and Sirius was smiling at all of them, heart happy and warmer than the Firewhiskey in his stomach. But behind that contentment, a small part of him looked at Happy James Potter (with his happy family and happy girlfriend and happy friends and happy Quidditch team) and wondered why Luck favored this one person so much. Sirius shook his head, used the alcohol as an excuse for the burning in his cheeks, and tried to smother the chill in his spine with the last of the Firewhiskey. He never told anyone that the ugly, petty envy never fully vanished.
When Sirius was 18, he had his first kill. It was War, and it was either him or them, but it still bothered Sirius - not that he had killed, but that he felt almost nothing when he finally had. It had been so easy, so disconnected. A single flick of his wand and there laid the man, dead. This Death Eater - this man - had a life, a home, a family. And yet, as Sirius watched the prone figure fall, the only thing he felt guilty about was that he didn't feel guilty about his death at all. Was he too much of a Black after all? After the fight, when everything had calmed down (died down, he thought, torn between wanting to cry and laugh at the absurdity of it all), Sirius helped bury the dead, burying with their cold lifeless bodies the burning shame he felt. He never told anyone.
When Sirius was 19, his father died, and his brother disappeared. In six short months, Regulus was also declared dead. Sirius had no time to mourn for any of them, not when his absence might mean more bodies to mourn for. Orion and Regulus had been supporters of the Dark Lord, and Sirius on the enemy side, but Sirius still couldn't help the stinging in his eyes when the declaration became official. Orion he cared about just as much as he cared about Sirius (meaning not at all) but Regulus - no matter how distant they had gotten - Regulus had still been the little brother Sirius had grown up and played with. Sirius was supposed to protect him. Wasn't he? Restless and tired and in the middle of a war, Sirius still tried to think of happy thoughts; trying to send a Patronus to Regulus, wanting to make sure if his stupid, stupid Death Eater brother really was dead (playing dead or not playing dead), and knowing that if he wasn't, a Patronus would definitely find him. Expecto Patronum, the room was dark. Expecto Patronum, Sirius shook. Again, again, Expecto Patronum, and finally, on his third try, misty silver light flowed from the tip of his wand and formed a hound. Sirius told the bright creature Regulus' name, but it didn't move - ironically, mockingly staying, when all Sirius wanted was for him to fetch (Regulus Black - Regulus Arcturus of the Most Ancient and Noble House of Black). Sirius never told anyone, but that night he had stared so much into his Patronus' eyes he started seeing a familiar pair of silver eyes that he knew at that point on he should only ever be seeing in a mirror.
When Sirius was 20, Harry was born. He wouldn't be exaggerating if he said his godson had been a ray of light in those dark days (and he had said that, multiple times, every time more truthful than the last). James had shoved him, mumbling he should get a kid of his own if he was going to hog his son, and Lily had laughed, saying she couldn't imagine Sirius as a father. They weren't trying to be insulting, but they didn't need to be for the barb to drive straight home. Sirius smiled and handed Harry back to James. He never told anyone, but he did want a kid of his own - only he was the last of the Most Insane House of Pompous Bastards, and he wouldn't want to pass on such cursed genes on an unsuspecting child.
When Sirius was 21, old habits resurfaced. Werewolves were a danger to society. Werewolves were Dark. Remus was a Werewolf, and he wasn't to be trusted. Sirius planned with the Potters, Peter, and Dumbledore, thinking of ways to keep the Dark Lord away from Harry. Dumbledore suggested the Fidelius, Sirius suggested Peter be the Secret-Keeper, Peter acquiesced, the Potters were settled, and that was that. Not one of them told anyone else. Or so they thought.
When Sirius was almost 22, everything ended. He had always been a tad dramatic, his friends knew that (he knew that) but this time he was sure it was true. The War had come to an end, the Dark Lord was dead - and so were James and Lily. James and Lily, who were both almost 22 - frozen and almost 22 but never, ever going to be. They won't be having any more birthdays, Harry will never have one with them, Remus wasn't a spy after all, and Peter- Peter was dead. Or at least he'd wish he was once Sirius caught him. Sirius ran after the bloody traitor, breathing harder than he ever had (breathing for James and Lily, almost alive, almost there, only just not quite), but Sirius didn't catch Peter. Peter caught him, and Peter wasn't dead. Peter was a rat and Sirius never had - never would have - the opportunity to tell anyone.
Now - like his earliest memory - Sirius had no idea how old he was, and he was alone and his knees were trembling and glass kept shattering on repeat in his ears. Secrets didn't matter, not anymore, not here in the cold dark, where nobody could hear him, where his body lay broken and his heart in pieces.
He was never the Potters' Secret-Keeper.
Every night, he tried to tell everyone - anyone - but nobody ever listened.
Note: Apparently, Sirius' birthday is November 3, and, if you'd recall, James and Lily were murdered on Halloween, and Sirius probably spent his birthday that year in Azkaban, alone in a place where happiness can't exist and imprisoned for a crime he never did and dammit now I'm sad over a fictional character's birthday