Disclaimer: I do not own Avatar: The Last Airbender
After all she has done for him, Mai thinks she has a right to feel betrayed. She waited for him while he was banished. Broke him out of jail (even though he had dumped her in a letter). Fought against Azula for his safety. And proudly went to jail with the knowledge that he was safe.
So when she returns from prison to greet her boyfriend, she's more excited than she could ever imagine. When he sees her he smiles, and her heart quickens uncontrollably. Yet his kiss that makes it shudder to a stop. The kiss is light and cautious. It's not what she remembers.
She tries to throw the thought from her mind, and she mostly succeeds. But in the weeks that follow the end of the war, the evidence is constantly shoved in her face. Zuko was a hero now. A Firelord who was a force for good. He had become a man in the few short weeks that she was locked away. He didn't need her like he used to.
However, it's when Katara is around that she knows for sure. She sees how they seem to gravitate towards each other. She notices every time their fingers brush. She secretly watches them gaze at each other when they think no one else is looking. And with this she can only come to one conclusion:
Zuko is no longer hers.
She can't deny it any longer. And as much as it hurts, Mai mostly feels betrayed. She did everything for him and yet he chose someone else. Someone good. Someone she could never be like.
So she writes a letter for him, like he once did for her. She expels all her emotions onto the page and locks her heart away when she rolls it up. Because she'll be damned if she ever feels this way again.