Mbluish,
Im so sorry you are going through this. I went through something like this about 17 years ago. I told my husband if he wanted to stay married he would have to go to counseling with me and stop all communication with her. He was willing but counseling was not helpful. We did find something that was helpful but I don't know if I can make a recommendation here per the rules, as I am new. I would suggest looking for intensive programs for marital issues. They can be costly but they help the participants to focus on what they really want and need.

We were able to save our marriage but if he had been unwilling to try...well Im not sure what you can do. Going to therapy yourself is a good idea. Find someone with expertise in dealing with infidelity. If he chooses to leave he may find that his fantasy is not as good as the reality but either way you need support now.