Have you read Laura Schlessinger’s “The Proper Care and Feeding of Husbands?” That might give you some insight into your H. IMO, his actions are no different than most men here. He wants affection and validation and does not feel he can get that from you. The OW is filling a need, but not all his needs. In turn, he is not giving you what you need to feel secure enough to meet his needs. It has nothing to do with a disease or depression.
He is depressed because of his marriage, not the other way around, which is the biggest mistake the MLC board seems to make. That mistake keeps them lock in the power struggle. Eventually one spouse or the other will prove out to be stronger (or more in denial). If it is the MLCer, then s/he will finally leave. If it is the LBS, the MLCer will come home with his/her tail between his legs, a remnant of a true man/woman.
Sorry, I think that board is pure poison. I think you are in the right place here. Read up on some of the many posts here. Pick up some of the relationship books we talk about. I hear you saying that your H is now working on himself, which is good. You say you are not the same person you were, but does that mean you have worked on yourself, or just hardened yourself? The first is good, the second is bad.