I didn't abuse him in the marriage..I think I just agree with some on these boards that we are sometimes responsible for our part in the marriage problems. I saw signs, and ignored them. Problems along the way that needed to be addressed, but I didn't face them head on. I put my head in the sand. Now, I am NO mind-reader either, and am only human, and WH was responsible for his part, including not putting his issues on the table. So I know I am not 100% responsible, far from it.
I am now reading a book (The Truth About Cheating, by M. Gary Neuman) and he says most men stray because the OW makes them feel wanted, loved and appreciated. I failed in one way or another in these areas. After 15 years, resentments build up.
My WH failed in them too, don't get me wrong.

And because of his family template, his different set of morals (lots of cheaters and divorces in his family and friend circle) walking out was kind of 'normalised' for him.

Walking away from his newborn was repeating what his own dad did.

Not believing in 'fixing' broken relationships is an idea he inherited from his mother, I think, who is a tragic and very unpractical/unpragmatic romantic (and disaster zone).