I'm so sorry you find yourself here, I can only imagine how painful your journey has been.
One thing that I wanted to point out to you, when you read about infidelity in DR, I think Michelle is talking more about people who have had one affair and are torn between an affair partner and their spouse.
Your husband is a serial cheater, and it sounds like he likes to have sex with any woman who will have it with him. I don't think he falls into the category that Michelle mentions in her book.
Yes, marriages can survive infidelity. But when you are dealing with someone who displays characteristics such as your husband, I think it's important to be honest with yourself.
I completely understand you want to save this, and that's fine. But at some point, you have to start valuing and respecting yourself.
And for the record, this woman he's communicating with, they're not just friends, this is you in denial. The best predictor of future behavior is past behavior. If it walks like a duck, and quacks like a duck... well you know the rest.