Hi, I'd like to help you with your question.
I am sorry to hear that your husband is doing this to you. By forming a relationship with another woman and pulling away from your marriage, he is putting his needs before yours and the marriage.
In order to deal with this, it is important that you let him know what you found out and tell him to stop the behavior immediately. Anytime a spouse strays outside the marriage it can cause strain to the marriage and undermine trust, which is essential to a good marriage. Your husband's relationship with this other person and his need to hide it is harming your relationship.
You mentioned that you tried marriage therapy. You may want to try it again. You need to rebuild trust and your husband needs to find out why he cheated and be sure he does not do it again.
There also needs to be clarification on some issues.
One, talk to your husband about what he did. Is your husband sorry? This is important, because if he does not take responsibility, recovering your marriage will be difficult.
Two, tell your husband to stop all contact with this other person immediately. He must do so as a first step to regaining your trust.
Three, see if he is willing to do anything to start repairing the marriage. Once this secret is out, he needs to be motivated to fix your marriage.
Four, make sure the two of you are talking about what happened. Your husband needs to be open and honest about what he did and let you ask any questions you need to. He broke the marriage vows and dragged you into another relationship. He needs to own up to it.
These are some of the most important issues you both need to be working on. If you do not want to go back to your original therapist, talk with your doctor about a referral to another therapist. Also, if you attend church, talk with your pastor. Pastors are often very good marriage counselors. Also, if you have problems affording therapy, try your local community mental health center. They can offer therapy on a sliding scale fee system.
The main issue is to rebuild trust. It can take a while and a lot of work, but you can do it as long as you both are motivated.
There are some resources that may help you:
Infidelity: A Survival Guide by Don-David Lusterman
Not "Just Friends": Rebuilding Trust and Recovering Your Sanity After Infidelity by Shirley P. Glass and Jean Coppock Staeheli.
Surviving Your Worst Nightmare: A Guide For the Betrayed - Patti Snodgrass
You can find these both on Amazon.com or your local library may have them.
If your husband will not stop his behavior or he shows low motivation to repair your marriage, you may need to seek individual therapy and decide how you would like to handle the situation. You will need support and input so you can make the best decision possible for yourself.
I hope this has helped you,