Okay, hopingforhope, I will try to be here for you, too. I was the one having the affair in my marriage so I can be here for her as the "other side".
I agree with what most have said. Get the intel, tell her that you know x, y, and z, and that you won't live in that kind of a marriage. Talk to her with love, but be firm on your limits. Understand that whatever kind of affair she is having, it seems real to her but it most likely is not. Her feelings are real, but reality is different. If she decides to continue the affair (I'm afraid she probably will because of the strong pull of it) then you start exposing. Start with a trusted member of your church, for example. Then, maybe a close friend. Then, maybe a couple of family members. The less people involved the better, but it will all depend on what she does.
This is not easy. Exposing haphazardly is easy, but not smart....the real work will come after she stops the affair. The "carrot" that was talked about is very important. She needs to see that her life with you can be great if she gives it the effort and attention needed.
