What am I doing to myself?

I'm clinging to a guy that isn't good for me, emotionally and physically. I don't know how to part from him... I've been friends with him longer than I've been dating him. He was so sweet as a friend, I wish he would remain that way. He turns into an ugly person when he drinks. Almost unbearable. He needs help... and I know I'm not doing him or myself any help by staying here. I'm just as sick as he is for staying in the situation. I tell myself things will get better or I just simply put the past in the past.. but this is one of those things that you can't just put in the past. He frightens me so much. I hardly even drink anymore because I know what the outcome between us is going to be. I fear when he gets drunk, because I know what he turns into. When he's drunk.. and he hears one wrong thing or whatever... that's enough to set him off. He can't accept me for my past when he's drunk.. but I accept him for his. He tells me it's bullshit that my past is the past, because if the past didn't matter then I wouldn't have shared with him the things that I did tell him when we were dating. To me, that just sounds ridiculous. Everyone has a past... take it or leave it. He's a bitter person. He holds in so much anger from his past that he takes it out on me... and by the time we're done fighting, I fall into his trap. He apologizes and I forgive him. I'm so weak in the knees. I'm letting myself go again. This is one of those things I was talking about in my earlier blog when I said that I thought life would get easier after going through each obstacle that comes to you. But it's not... life keeps getting harder. Not all the time. My life right now is like a roller coaster.... ups and downs. This is most definitely a down. I just want someone who will treat me right and respect me for my past. I want to believe so much that he can change, but we all know that the only way he's going to change if he seeks help himself... which is very unlikely. And I hate to have doubt in him, because I believe so much in people. I'm just so attached to what a beautiful relationship we had while we were friends, and idk if I'm strong enough to remain his friend if anything did happen. I think I'm in love with who he was when we were friends, the sweet side. All I see in him now is anger. Don't get me wrong, he can be a total sweetheart, but that's no excuse for the way he treats me....