Get the book "Not 'Just Friends'" by Shirley Glass. It should help you understand why, or how, the affair happened. In fact, it may even help explain it better than your husband ever could because he may not fully understand it.
There are other good books too. It can take a long time to get to a place where you can let go. And some people take longer than others. I believe from what I've read, a general time frame is about 2 years. With my husband's first affair it took me about 5 years to fully get over it. That's when I finally never thought about it, and when I did, the pain was gone. I also trusted my husband again. Although I never was naive enough to believe it couldn't happen again. And that, I think, was good.
