Dear Mai,
It had only been a few weeks and now he was going away...again. He had to keep telling himself that this wasn't like before, he was coming back, he wasn't leaving her alone forever. He was trying to make sure they could have the happy future they had both envisioned back when they still held some semblance of innocence all those years ago, before he was marked a failure, before he was banished. He knew it was the right choice, that this was the path he had to walk.
I'm sorry you have to find out this way,
This was how it had to be, there was no other way. He knew he'd never forgive himself if she died because of his choices, that would be a future far worse than the past he had tried to run away from for so long. At least this way there was hope, if she died, he lost even that. Zuko placed the scroll onto her pillow, the same one they had lazed against only days before, the same one she would put her head on tonight after she found his letter, the one that would catch the tears he made fall from her pale gold eyes. He couldn't face her though, she'd want to come and he knew it would only take a matter of time for him to succumb to his selfish desires and bring her along. Having a familiar face by his side would make everything so much easier, but nothing in his life was ever easy, he wouldn't let this be an exception when he knew the consequences. Leaving her was the right thing, and facing her was out of the question, this was the best he could do for her right now. Asking her to come was tempting, putting his cloak and swords back and burning the evidence that the thought of defecting to the Avatar's had ever crossed his mind even more so, but he couldn't turn back, he knew what his role was now. This was the right thing to do, he was just sorry that this first time he did something right it would hurt her.
but I'm leaving.
Zuko walked silently through the empty streets, heading for the underground bunker he knew his father was hiding in during the eclipse, the last thing he had to face before solidifying his betrayal and leaving his home once again. But now it was his decision, it was heart telling him what was right and wrong, his sense of honor that was dictating his moves. He continued moving, there was no other option, he had to do this. He would the Avatar and help him defeat the tyranny his family's rule had created and the poisonous hate it had spread to every other nation, and then he was coming back. He wasn't leaving her forever, this was the right choice, of that he was sure.