I think Mgran and dragonfly said what I was trying to say, but couldn't. Perhaps when you write him the letter (maybe an email would work better?) you could tell him that he can't continue with the path he's taking, and that he needs to get help, or you will be forced to leave the relationship to take care of yourself.

I also don't think therapy would a bad idea for you right now. It might be helpful to have a third party knowledgeable with what is going on in your boyfriend's head to bounce things off of. They'll probably also have ideas on reopening communication and how to talk to him about going to therapy himself. Perhaps if your boyfriend sees that this issue is important enough to make you go to therapy, he'll start to reconsider his stance. If he is able to visit you at all, perhaps you could bring him to a session. At the very least, if it turns out that this relationship can't be save, you're therapist will be able to help you through the break up and teach you how to make yourself your number one priority in life.

I don't think you should just trash the relationship you've worked so hard on. But I do think you need to make it plain to him that you need to take care of yourself and that if there isn't some sort of improvement or an attempt to work on himself, you'll be forced, out of necessity, to move on. Granted, he has to want to get better on his own, but maybe he simply needs a push to show him that his behavior is hurting the ones he loves to get him so he feels he needs to change also.

Good luck! Everything is going to take time, but keep us updated! And remember to take care of yourself!