Unfortunately sometimes it is a journey or a path someone has to take before they realize what they lost. Doesn't help us - but it is true.
Amazing how scripted and rehearsed all the things sound defending these affairs. For something so chaotic and destructive they all say remarkably similar things.
We can only focus on the things we can control. Ourselves. Our relationships with our children. Our own happiness. It sounds like giving up but I don't think it is. Just letting go and seeing how things work out.
From experience I say you can drive yourself crazy focusing too much on it. I can think of numerous rational arguments you could make to your H but unfortunately he isn't listening to rational thoughts right now.
"You can't reason someone out of a position they didn't reason themselves into."