Others have written down everything I wanted to so just a few things I can add:
My xABF was kind, supportive, loving and gentle all the time. My Dad was like this too and he was an alcoholic too. That's why I've left my xBF. I knew that nothing will change (I was right, he's dead now, he died because of the alcohol and he was SO young).
I loved my Dad and his alcoholism hurt me even more because of this. Because he was kind and loving, I found it really hard to be mad at him when he was drunk. It was killing me to see him every day struggling with his drinking, shaking in the mornings, sick all the time, being in the hospital because of alcohol-related illnesses and finally losing him (cirrhosis). I wanted to be free of worry, I wanted to be far away from him and I wanted to be near him and stay with him. It was a terrible situation to be in. That's why I've left my xABF - I didn't want to do it all over again, to hurt like that again. I sometimes wished they were abusive and violent so I could hate them...but I loved them with all my heart and I had to watch them fall apart.
You say "the thought of starting over seems unbearable". I used to think like this too. But after leaving him I've met a gorgeous man who is my loving, kind and not alcoholic husband now and we're really happy. I've given myself a chance to be free and happy and I've never regretted it.