For what it is worth, I had sex with other women whilst my wife and I were separated. Partly out of revenge I guess, partly to keep the smell of desparate off me and to see if I still "had it". All ridiculous things really - and I still felt dreadful.
If your H has been involved with this woman due to feelings of "revenge" I can understand the motivation but I am not excusing it.
What I didn't tell Daddy Long Shanks was that in addition to being quick to forgive most of the time (for trifling things), I'm also quick to anger and it took a long time to really forgive my wife. A "trifling thing", by the way, is anything you won't give a damn about 1 year from now.
I really hope your H sees sense and takes responsibility for his part of your M's demise. There was no excuse for your A but his treatment of you made you vulnerable to one and he really does need to learn how to meet your needs, starting with knowing what they are in a language we men can understand.
My wife's sister has just dropped the bomb on my BIL and they have four young children. This is what has led me back to the board - I fear the Monster of Complacency like you wouldn't believe!
Good luck with this!
best, GH31
Me: 46 W: 46 T: 23 M: 20 DS12 DD11 DS5
W left: 01/28/08 Discovered OM: 02/26/08 W back for 9 days: 04/08 W returned 05/21/08 EA/PA - 01/08-07/09 W's MLC 2008-2014 (realised this much later)