Your husband is embarking on more bad decisions it sounds like to me. He is cheating with this woman. Its time to do things right and encourage her to be up front and honest from the get go. There is so much possibility if everything is based on honesty, integrity, consideration for others. There is nothing but deciet, pain, sadness and loss with cheating. If I were you I would be very clear that his relattionship with her right now will destroy all of your lives if she does not set this straight.
I would suggest that you and he do a tag search for "cheating" here and read up on what happens when you cheat. That way both of you can support her.
As for where you are at? I think that starting where I suggested is the first order of business. After that, see where you are at and see how you feel. Doing a tag search for "foundations" and "lessons" might help with ideas on how to start out with a foundation that works this time.
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