I understand your anger and you have a right to feel what you feel. On the other hand, expecting a well thought-out, heartfelt apology from someone who is currently withdrawing from drugs (it takes months!), confused and worried about his future and ability to stay sober, feeling sorry for himself, and having just been rejected by his love interest, I can't say that I am surprised by his response to your letter.
My recovering addict son would probably respond the same way -- and I also know that the shame he feels about his past actions is the heaviest and most crippling burden he carries. Does he talk about it much? Hell no. But I know that about him. I know that he is working hard to get to that place where he CAN talk about it....where he does have the strength to apologize properly. He knows it's the key to recovery....but it's hard. All his life he's conned and manipulated people....he has a LOT to apologize for. It may never come.
Sometimes I am pretty angry, too. But mostly I am sad and disappointed when I don't see that healing taking place. I am forever hopeful and pray that someday he will be healed of this spiritual malady.