Welcome to our unpleasant world. Since you asked, here is my advice to you:
1. Visit this website daily. Read and read about what others are doing and what they have been through. You will find a lot of similarity.
2. Keep this thread updated. You will only get advice and help if you keep your thread current.
3. Do not tell your husband about this website or anything else that you are doing to prevent a divorce. Hide everything you are reading in a secure spot. This is very important.
4. Although Hurtin mentioned that you may dive to the depths of hell, do not be afraid. Your fear will doom your marriage. Be strong, confident, and positive. Be chipper and optimistic. In general, behave in the opposite way that you feel. I know, I know, this is darn difficult. But your husband will be repulsed by negativity, grumpiness, moodiness, and bitchiness. So you gotta be a person he wants to be around, even though, right now, he does not want to be around you.
5. This may seem like an impossible goal right now, you need to reach the point where you no longer depend on your husband's love and acceptance for your happiness. Right now, you are saying, "I can't be happy without him!!!" But you can. Work on getting to this place.
6. The relationship with his girlfriend is doomed. Keep that in the back of your mind.
7. Don't conspire against him, or against them. Anything you do to drive them apart will only push them together. If you can't control it, let it go. (this, dear, is the most difficult of them all).
8. Involve yourself in activities outside the home and outside the marriage. Join groups (support group for parents of autistic kids, divorced/separated, prayer groups... anything). Go get a life outside the marriage. It will have a double-pronged effect of showing your husband you have "moved on" and it will take your mind off of the pain of what he is doing.
OK, you have your homework. These eight things are the foundation of what we do here.
It ain't easy, girl, but I know you are up to it. I know this because you had the strength and wisdom to hit the "NEW TOPIC" link and type: "Question-Any good basic advice for me?"