I mean realistically what are the chances that this affair will last?
I would say zero to none. However, he doesn't want to work on the M. That's the thing! It's so much easier to start fresh......and with a fresher gal. I don't know anything could hurt a hurt worse than a spouse dumping his W for a younger version. Unless, it would be for her to get a younger man (ha). However, I don't advise that you get into any type of revenge tactics.
Stick with the board, b/c you'll get great support here. There are many others on the board who know exactly how you feel.
It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!