Hon, this is exactly the sort of time to do something for yourself. You don't have to have a front row seat to his drama. Go out to the mall, get your nails done, go for a walk, drive the car with some favourite music on.
Have you tried Al-anon? Not only can you learn how to deal with these situations and much more besides, but you also have people to call in times of need.
And he hasn't "slipped", imho, he never really stopped in the first place. That's the reality. I know how hard it is to see someone you care about put themselves through this. I always shudder to think what kind of pain they are running away from that makes this a better option.
You can't help him, but you can help YOU.