So TVTropes has said How I Became Yours would be brilliant if it was a tragedy/horror from Mai's POV and this happened.
When she first suspected it, she pushed it away. I'm being silly, paranoid. There's no reason to believe it. Even if he'd been close to that Water Tribe girl she and Zuko had an unbreakable bond, right? Childhood friends, hopelessly in love as teenagers, she'd saved his life and he'd fought jealously against guys who hit on her. He hadn't objected to their arranged marriage, even seemed happy about it.
He just misses her as a friend. He misses them all, now that we're living here and they're traveling the world. Every day she would remind herself of this, and anytime he smiled or held her hand or kissed her, she believed it.
He loved her.
Right?
When she found the letters from the Water Tribe girl, she brushed them off as the lovesick longings of a teenager. Maybe she wasn't as happy with the Avatar as she seemed, but her having feelings didn't mean Zuko returned them. He probably kept them just to humor her, she was still a child and sooner or later she'd realize what she felt was infatuation, not love.
Love was deep, strong and enduring. Not flowery declarations of need and desire, not tears smudging the ink that read I miss you, please come back to me, my life is so empty without you. She didn't remember the Water Tribe girl being so dramatic, but maybe it was just a phase. And Zuko still didn't feel the same. He loved her. Mai. His wife and partner for life.
They loved each other.
Right?
She hid the letters from Zuko, hoping if he never found out he'd never have to break the girl's heart. She had to get over this silly crush on her own.
And every day, she told herself Zuko loved her. He loved her, he had to. Even when he no longer held her hand, rarely smiled at her, only kissed and touched her out of obligation, he still loved her.
Didn't he?
One day she discovered letters, letters Zuko had written to the girl but never sent. Letters proclaiming love, passion, longing...feelings he'd never expressed to her.
She should have burned them. But by now she was desperate for any sign that he still loved her and that this Katara girl was just a summer fling. She kept them, crossing out Katara's name and pretending they were for her eyes only. I love you so much, every day my heart aches for you more and more. You and I were truly meant for each other.
Then that letter arrived. The girl was pregnant, and it was Zuko's child.
Maybe it was desperation, knowing the Fire Nation's discovery of the child would cause scandal, political upheaval, destroy Zuko's approval rating. Maybe it was bitterness, knowing that she'd miscarried her and Zuko's last attempt at an heir. Maybe it was anger, Zuko had cheated on her after all she'd done for him.
She discovered a poison that would take care of the problem, laced a gift of fruit from Zuko to the girl, and tried to forget about it. She'd taken no pleasure in what she'd done, but she'd had no choice. Even letting the girl have and raise the child in the Water Tribes was too risky, what if they looked like Zuko?
Weeks later, Azula, having escaped from the mental asylum, found out about it and tried to blackmail Mai for a seat on the senate in exchange for her silence. This time she had no compunction about poisoning Azula and leaving her to die on the street. Azula had tried to kill her once, after all, and then left her in prison to rot.
She kept the letters, though. Her fantasies were all she had left as her marriage fell apart and she could do nothing to stop it. The truth had settled in like a fog on the horizon, smothering the last rays of hope. She lay beside him, fighting back tears every night and trying to block out the moans of Katara, Katara.
His love was for someone else, but she still loved him. And she would do anything for him, even if it meant sacrificing an innocent child.
One day, he came to their chambers to see her. His uncle had found the letters thanks to her maid, Lin, and he exploded at her in rage.
She panicked. She denied it despite the little voice chanting liar, liar inside her head. I don't know what you're talking about, Zuko, I never saw any letters, I-
Pain exploded in the right side of her face, she felt herself being lifted and then thrown to the floor. No! Zuko, why...
"Liar!" he spat. Screaming, ranting, raving, all sorts of horrible names and accusations. She bit back tears, trying to keep calm despite the rage growing inside her.
"I did it for you!" she blurted out. "Do you realize the scandal a mixed-race bastard would cause? If they were ever found out, the whole Fire Nation could erupt into a civil war! I had to protect you from such a thing!" She looked into his eyes, desperately searching for some shred of understanding. "Zuko, for the sake of your position, for our marriage-"
"I never loved you!"
Something inside her broke. I never loved you. She knew, she'd known all along, but to hear it from his own lips...
He slapped her again, declared her no longer his queen and slammed the door behind him as he left. Her cheek throbbed, she tasted blood, and she felt sick. Holding back the urge to vomit, she picked herself up and hobbled to the wash rooms.
I never loved you.
Memories of them playing as children, kisses in the sunset, breaking up and making up, their wedding day, his promises to love and cherish her until the day they died. Fake. Everything was fake, none of it real, he never loved me, no one ever loved me. Mother, father, Tom-Tom, Azula, her uncle...Zuko was the one person she'd thought she could trust.
He and General Iroh left that night, and all Mai could think about was revenge. That girl had destroyed her marriage, her hopes and dreams, and she wouldn't be satisfied until that girl's life lay in tatters just like her own.
Sho wasn't her brother by blood, but they'd known each other since she was a toddler and he'd always referred to her as "baby sister". Mother had never approved of their friendship, but Mother didn't own her any longer. She was no longer the good, silent "lady" everyone wanted her to be.
Sho had helped her poison Katara, and later Azula. She came to him, and he was all too happy to assist her.
They found the happy "family" living in the Bei Fong estate; while Sho kept the girl's brother distracted she sought out her rival. The little brat who's destroyed everything she held dear.
"He never gave me flowers. He never said such sweet words to me. I gave him everything, I gave him my heart and soul and you manage to pry him away with a few pretty words and a bastard child!" She heard herself laughing cruelly. "I made something to take care of the problem. Something just for you to eat...and I took great pleasure in doing it."
With those words, the last threads of her sanity had snapped. The girl wore blue but Mai only saw red, throwing knife after knife even as the girl dodged and leapt from the window. Come back here, you little wretch! She would have her revenge, she would paint the courtyard with the girl's blood, she would-
Ah!
Suddenly she couldn't move. The girl had her cornered, her hands held in a strange way and as they kept moving Mai felt more and more pain, her muscles twisting and bending and her bones snapping and her vision red and ahhh Agni stop make it stop please make it stop
This was it. Her punishment for her unsavory deeds, deeds she'd done to protect a man who'd never loved her. She'd sold her soul to protect him and now she was dying, she knew it. Slowly, painfully.
The girl's angry face taunted her, and she closed her eyes, pretending it was Zuko's smiling face.
I love you.