Your story reminds me of mine SO much. It drives me crazy that don't understand his actions or how he could be so unhappy "all of a sudden" I'm not sure how much has to do with alcohol, midlife crisis, or just unhappy with his marriage.
Its a painful thing to go through. The only thing that would be harder is the loss of a child.
Al Anon helped me stop obsessing about all the what ifs and why that were never going to be answered to my satisfaction. I got out of his head and learned to focus on me. I was also amazed at how many people were there to lend me a hand when I needed it. And I discovered that it was okay to ask for help and accept help when I was at my weakest.
Life just sucks sometimes. And it may not feel like it will ever get better...but it does. Maybe not in the way you want it to but life will go on.