Dear You,
I don't know if an apology is what I should do in this situation. Probably because I want one from you. For having me believing you liked me like I liked you. I really believed it. You don't know that though. And if you did then I might be at a loss for words. Even if you think it wrong, I do owe you something. I never told you I was leaving. I know that many people would tell I'm not at all at fault here.
But I think if I just say it...it might relieve some of this tension and hurt. So...
I'm sorry.
Sincerely, Me