I do hold resentment and I'll be the first to admit it because I did not deserve this!

Of course you didn't. But life isn't fair, it turns out.

We were newlyweds, happily married, even my H will attest to that! Our M was great!! That is whay I cannot understand!

It's difficult to understand because it has nothing to do with logic. It's something like this: Your H was vulnerable, and in his contact with what's-her-name, discussing emotional issues with her, a bond developed... that affects brain chemicals of attraction and infatuation and being vulnerable, he didn't stop it. It was pleasurable and he sought more of it. He's 100% responsible for that choice... but hey, that's in the past, he came out of it... now you've got something we all want, a remorseful ex-WAS wanting to work on the relationship. Not getting back into a situation like that is part of what you two will work on.