LadyBug, I have experienced the same thing with my AH. We had a lot of really good years. Then over the past few he has become almost a different person. He's the opposite of who he was in so many ways. I know what you mean about it feeling like a death. I really grieve for the many my husband used to be and wonder where he went.
I am reading a book that I think you would get a lot out of as it delves into these questions you're asking. It's called "Under The Influence" by Milam and Ketcham. It focuses on the physiological aspects of alcoholism. It is REALLY helping me understand what is going on with my husband, and why. I'm wishing I'd had this book months ago. Anyway, check it out, it talks a lot about these personality changes and what is causing it and how the body recovers from long-term alcohol use.