I dont think Zuko was, or ever will be, truly in love with Mai. Does he care about her? Yes. I think that, just like with almost everyone else on the planet, he wishes the very best for her. But love, I believe, is a very strong word, as is hate, and it doesnt apply to the Maiko relationship. Not from Zukos point of view. You cannot be in love with someone who is on such a different level, does not understand you, and is in love with the person that you used to be, with no knowledge of the person that you have become.
But Zuko left Mai a letter! That means he loves her, right?
Youve got to be kidding me. Zuko knows Mai well enough to know not only how much she feels about him, but how she would feel if he left without any explanation (although, as witnessed in The Boiling Rock: Part 2, Mai wasnt exactly thrilled about Zuko leaving her a letter, either). Hes not a completely heartless jerk; sheesh, give the poor dude a break.
You could, of course, consider this evidence that Zuko doesnt love Mai enough to extend that sort of courtesy. He had plenty of time to tell her what he was doinghe could have let her know. She ranted at him about not doing so, remember?
All I get is a letter? You couldve at least looked me in the eye when you ripped out my heart.
But he didnt know he was going to do it! And he had to talk to the Fire Lord!
True, but in the time it took him to write the letter to Mai, he could have just walked over to where she was and told her everything. The entire invasion hadnt even begun at the point in which he began to write that letterhe had plenty of time to tell Mai what he was doing. She lived, like, right across the street from him. He just needed to walk out and deliver the message to her. Its really not difficult to do, and really, it could have given him some exercise.
But the entire palace city had evacuated!
Then find Mai before they plan to evacuate and tell her. Im quite sure that Zuko was convinced that his decision was what he wanted to do by the end of the war meeting. He did not have to write that letter; Mai herself already confirmed that.