Hi, I'd like to help you with your question.
It appears that your husband is in this situation only for himself. He is hurting you and allowing his mistress and his life with her mesh with you and your marriage with him. This is causing you undo stress, which you do not need on top of struggling with cancer.
The first step is to decide if you want to stay in this marriage. If you do, then several things need to happen:
Anytime a spouse strays outside the marriage it can cause strain to the marriage and undermine trust, which is essential to a good marriage.
You mentioned that you tried therapy. You may want to try it again, and this time with your husband. Both of you need to go to work this out.
There also needs to be clarification on some issues if he wants to come back and make this work.
One, is your husband sorry for what he is doing? This is important, because if he does not take responsibility, recovering your marriage will be difficult.
Two, your husband need to stop all contact with this other person. He must do so as a first step to regaining your trust.
Three, has he done anything to start repairing the marriage?
Four, are the two of you talking about what happened? Your husband needs to be open and honest about what he has done 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. Talk with your doctor about a referral to a therapist. If you attend church, talk with your pastor. Pastors are often very good marriage counselors. You can also search on line at http://www.bacp.co.uk/.
If your husband is unwilling to leave the other relationship and work on your marriage, you have two choices. Put up with the situation as it is or end the marriage. Putting up with this is going to cause you stress and affect your health. You do not need to be between your husband and his mistress or be treated so poorly. Ending the marriage is a way to begin the grief process and begin to heal so you can move on with your life. It will give you control over your life and hope for the future.
I hope this has helped,