He needs help. Not to continue my analogy, but he is a drowning man. Be careful, he will take you down with him. It's okay to love him, but do it from a safe distance.
He can't be what you need right now. Separation doesn't mean a divorce and I am sure you know that. But, in my opinion you need some space from him and he needs to get his head on straight. Right now my guess is that he hates himself for all sorts of reasons. There is no way for him to find a way to make himself happy and he is looking for someone to do it for him. The other woman was his attempt to fill some hole in him and it didn't work. He is merely flailing about right now.
You are going to have to be strong and determined. I wish I could tell you an easy answer. The only advice I have if for you to encourage him to find individual help. He needs it. Stay away until he shows you he is willing to work on himself. It's not a make or break time. It's a "he needs help" time. Let him get help and then work on the marriage.
