I actually had all of these notes typed out on my computer but since I can't use it, I'm going to have to rewrite everything.
So let's start with this: Thanks so much for everyone who read and commented on this story! I was surprised with the amount of support I got for it and it was truly a pleasure to write this!
Now I'm only saying this now but Lost and Not Forgotten is the kind of but not really sequel to For An Everlasting Smile. The reason I didn't say so before was because I didn't want anyone to be dissuaded in reading this story because they thought they had to read something else first. Doing so isn't necessary and there are only a few call backs or hints to For An Everlasting Smile that don't take away from the story if you haven't read the other before.
Some one asked me a question regarding to Papyrus falling in love with Sans in the "normal" timeline. I neglected to give an answer until now. In the "normal" time line, Papyrus not only doesn't fall in love with Mettaton, but he also doesn't fall in love with Sans. The "normal" timeline is considered to be the one in the actual Undertale game. So the fact that Papyrus didn't fall in love with Sans was for lack of changes to the original timeline. He hadn't done enough to get Papyrus to love him when he made the mistake of prematurely introducing him to Mettaton.
Speaking of, Papyrus didn't have a lot of time as a narrative POV and there's a specific reason to that. This was primarily a Sans story and Papyrus's POV was to remind everyone that he doesn't remember shit from the bygone timelines Sans clings to. If you've noticed, there are no moments where Papyrus feels like he's reminded of something when Sans does or says something. The whole point of that was to establish that Sans is very much alone with his memories.
Now, enough blah blah! ENJOY!
Five years passed excruciatingly slow. Sans remained alone for a vast majority of those years, only reconciling with Grillby the year before. The flame monster had yet to forgive him, and Sans doubted he ever would, but he was kind enough to decide to simply move on and look forward. It wasn't like this timeline would last forever anyhow. Soon, everything would reset and Sans could live the timeline he had initially intended to.
"You seem chipper today." Grillby noted as he cleaned a whisky glass.
"yeah, i've been waiting for this for a while." Sans smirked as he sipped on a bottle of ketchup. "it's why i ordered this for a change."
"Going sober? Shocking for you." Grillby snorted.
"heh. just treating myself." Sans shrugged and stared emptily in front of him. "today's going to be a horrible day. the worst of our lives."
"And yet, you're still in a good mood." Grillby noted, "You sure you didn't add any vodka to that ketchup while I wasn't looking? Only reason I can think for you to be so happy after such a grim prediction."
"nah, i promise." Sans smiled and sipped at the bottle again.
"Then does this have anything to do with Papyrus?" The flame monster asked, "You did ask him to meet with you in a few hours, right?"
"yeah." Sans confirmed.
During the past five years, Sans had only seen Papyrus a handful of times – even going two full years without seeing nor hearing from him once. But, every time they did meet, the taller skeleton tried to convince him to let him come home. Sans refused him every single time. Now that he thought about it, maybe Papyrus was hoping that, today, he would change his mind. After all, this was the first time in five years that Sans had invited him to do anything let alone agree to spend time together. The other times before had consisted of instances where Papyrus forced himself into the smaller skeleton's life or accidentally bumped into him while working.
Papyrus was still with Mettaton. He had lived with Undyne for a few months, but then fixed his relationship with the robot. The skeleton always wondered what that had been like. Had Mettaton slandered Sans or taunted Papyrus with how he had been right during their fight? Had Mettaton simply opened his arms and heart and comforted his brother benevolently? In the end, despite his curiosity, Sans didn't really want to know. Ignorance was bliss, after all.
"Cheer to that." Grillby said, raising the now clean glass.
Sans smirked and raised his bottle. "but first i've got to meet with an old friend." He added grimly, though the flame monster didn't seem to take note of his somber tone.
"Old friend?" Grillby repeated, amused. "I can't quite fathom who that would be."
"you've never met them." Sans replied, "and hopefully you won't ever."
"You do love your secrets." Grillby commented with a bit of resentment. Sans didn't blame him.
The skeleton glanced at the clock hanging on the left of the bar. There were two hours before the human finished rampaging through the Ruins and murdering Toriel. It was sad to know his friend would meet her end betrayed by a child she had genuinely loved in their short time together. There was no solace to be found in the resets either. Where Sans would get the opportunity to turn his life around, Toriel would find herself once more in lonely ruins mourning the deaths of her beloved children and scorning her husband. Her life was truly tragic, but at least he could brighten it a little with silly knock-knock jokes.
"it's part of my charm." Sans joked and Grillby laughed.
"Of course." He agreed, "The whole dark and mysterious joking skeleton monster angle really works for you."
Their conversation was interrupted as Dogamy and Dogaressa entered the bar, taking their usual seat near greater dog. Grillby excused himself as he brought them their usual order of dog treats laced with a drop of grape juice for a nice, nearly deadly buzz. They barked their happy thanks and in just a few short minutes, Grillby was continuing their conversation again. Sans spent the remainder of his two hours in such a fashion. It was nice and comforting. It helped him mentally prepare himself for the impending apocalypse.
"welp, looks like it's time for me to go." Sans sighed and jumped off his stool. He stretched his bones, sighing when a few of them popped.
"Are you going to make it in time?" Grillby asked, glancing at the clock. "You've barely got ten minutes before you're supposed to meet with Papyrus."
Sans smirked and winked, "don't worry. i'll just take a short cut." he said and teleported immediately after.
The skeleton found himself in the forest surrounding the entrance to the Ruins. The atmosphere was gloomy and tense and he took note of the faint presence of dust lingering in the air. It was as expected; the human had commenced another genocide. And just as he made the conclusion, the doors to the Ruins opened with a thunderous rumble. Sans peaked from his hiding spot and saw the human emerged with a sickly grin covered in dust and blood, clutching a toy knife.
"Sansy!" They chirped as they wandered down the path. "I know you're waiting for me. You always are. Come on, let's stop this silly game."
The human carefully stepped over a twig and, when they were far enough, Sans snapped it in two with his magic. They whipped their head back insanely fast in response, hoping to catch a glimpse of him. Their reaction time was astounding and Sans could feel himself start to sweat. He didn't remember this part of the timeline as well as he would like to. His memories always blurred after the human murdered Papyrus and he could never remember how his own battle with the human transpired. That being said, the success of his attacks depended on a low reaction time and this was already a bad sign. He may have wanted a reset, but Sans wasn't charmed at the idea of a brutal death.
The human continued to venture down the path. Lost in his thoughts, Sans accidentally let the human wander much farther away from him than he would have liked and so he teleported behind them with the intent of then teleporting to another location. However, the moment he landed on the path behind the human, they wiped around and snatched him by the wrist.
"Got you." They chuckled darkly.
"…you're fast." Sans commented. Chills were running up his spine.
"Of course." The human boasted proudly, "I don't have a choice if it means beating you. Speaking of, I can't wait. I've gotten pretty good at killing you, you know. Do you remember?"
Sans glared and pulled his wrist out of the human's grasp. "you're as despicable as ever." He sneered and the human just laughed. "i don't understand why i'm still glad to see you."
The human immediately stopped and looked surprised. "Oh, Sans…" they continued with a teasing smile spreading on their face, "Have you fallen for me? I'd say I'm flattered, but that's really disturbing when you think about it. Love at first murder isn't a thing."
"you're disgusting." Sans hissed.
"Now that doesn't sound like something someone who's in love says." The human noted, tapping the toy knife pensively against their bloody chin. "So if it's not love, then why are you happy to see me? Have we become murder friends?"
"shut up." Sans growled and shoved past the human, "it doesn't matter—"
The human aggressively grabbed him by the hood of his sweat shirt and pulled him back, pressing the toy knife to his throat. Sans was admittedly taken by surprise but still possessed the power to kill the human instantly due to their low hp. However, he wanted them to reset and so neglected to do so just yet. He needed a full reset, not a partial one.
"I think it does matter, Sansy." The human hissed, face inches from Sans's face. "You see, I love hurting you. I get off on it, really. It's the only thing that brings me an iota of joy in this horrible, horrible world. So I need to know why you're happy to see me. It's important for my happiness."
"screw you." Sans hissed and tugged himself out of the human's grasp.
The human seemed absolutely infuriated. For a moment, the skeleton thought they would attack him and maybe they would, but they were interrupted.
"Sans?" Papyrus called nervously from a few feet away.
"Papyrus…" The human muttered curiously. "You look…different." Their face scrunched up in confusion; "Was there an update I didn't know about?" but then they caught sight of the pink charm hanging around his wrist. Sans froze when he noticed little figures of Papyrus, Mettaton, and read hearts attached to the metal bracelet. "That's different…this has never happened before…I don't—" The human looked back at Sans for answers when it suddenly dawned on them. "You…you did this. You changed the timeline."
"no, i—" Sans desperately tried to lie.
"Yes! Yes! It all makes sense now!" The human excitedly exclaimed, "You fucked up! You made a mistake! You're excited to see me because you think – or you know I can fix it somehow!" They thought for barely more than a second when they stumbled upon the right conclusion. Sans felt himself breaking under the unfolding disaster. This wasn't supposed to happen. "The resets! You want a reset!"
"Sans, what's—" Papyrus tried.
"Shut up!" The human shouted harshly, launching their knife at Papyrus's face. Sans barely reacted in time to deflect the projectile with a magic bone. The human hardly seemed to care. "I don't know how you fucked up, but it must have been pretty terrible if you're actually happy to see me."
Sans knew where this was going. "don't you dare—"
"What? Didn't you once beg me to give you all a happy ending?" The human snarled devilishly. "It would be cruel to everyone else to take back your wish now that I'm ready to give it to you."
Sans lost it. He was overcome with a desperate sort of anger and he couldn't think of anything else to do other than summon his blasters and annihilate the human so he did. Papyrus shrieked and backed away in horror. He hadn't known about the degree of Sans's power and was daunted by it as he should be. The powerful attack had drained the human of all their hp but their determination kept them around for just a bit longer.
"You're so stupid, Sans." The human laughed as their heart threatened to burst at any second. "I'm going to reset and I'm going to bring you all to the surface, give you the happy ending you all wanted. I'll even face my mortality and let myself die as I was supposed to, destroying the reset phenomena forever. And you'll keep living with your dumb mistakes until the end of time."
"why are you doing this?" Sans gritted through his clenched teeth.
The human smiled sweetly at him. "Because you're my favourite, silly."
Then their heart shattered and they reset.