I understand exactly what you're going through and am in the same situation. I think I finally made the decision to leave my AH of 13 years (our anniversary is next week, and I'm devastated.) AH goes through periods of sobriety where he is the wonderful, loving man I once knew, but he continually relapses, and each time he does it's worse than the last. This time I found out he was unfaithful while on a bender, and that he's been texting other women. He says it's the disease that makes him do these things, and is currently 4 weeks sober, but I don't know if I can keep living my life waiting for the other shoe to drop.
Leaving AH will be the hardest thing I'll ever do. I've loved him since I was 15 years old, and when he's sober, he's my best friend and the person I want to spend the rest of my life with. But I'm getting less and less of that man every time he picks up the drink again. My therapist told me last week that I may have to accept the fact that the man I love no longer exists.
I, too am grieving for what could have been and am terrified that I'm making a mistake. My biggest fear is that he will get sober and I'll spend the rest of my life regretting my decision. But I know that I should want sobriety for him anyway, whether we're together or not. I also have a daughter who is devastated that we are separated. I've explained to her that daddy has a disease called alcoholism that makes him make bad choices and that's why we're separated. She told me that if daddy is sick I shouldn't abandon him.
Anyway, just wanted to let you know you're not alone. What a terrible, tragic disease...