twl142: I'm jumping in here, although I'm new to this forum.
Prayer is a game changer! Only God can change your husband, but that doesn't keep you from praying for him.
My situation is SO similar to yours, only my husband IS a Marriage/Family therapist who thinks he wants out.
I would encourage you to read the book by Dr. James Dobson called Love Must Be Tough. It specifically touches on "opening the cage door" for the spouse who feels trapped in the marriage and specifically targets infidelity.
At present, I don't think my husband is in an affair but it struggling with PTSD from several military deployments. I sent him the letter I described above that basically says "I love you and don't want to think about living with out you; if you want out, you're free to go." That was a month ago, and I've received no response. I basically put the ball in his court. It's something for you to think about!
One thing I've learned is "the harder you go after him, the more you push him toward her." All of this is counter-intuitive, and I need you to know that I'm healing myself by writing this to you.
There are several ezine articles online that are excellent, and I would recommend doing a search for Leslie Cane. Her articles are very pointed and address some of what you're going through.
Pray without ceasing. He needs it; you need it; we all need it!!
I'm certainly not a counselor, but I am a military Chaplain's wife. I've listened to many wives over the years and never thought I'd be in this predicament. If I can help you, I'm glad to.