Honesty. In my experience, honesty is the best policy. If you feel or "sense" that there is infidelity then you should confront your partner in an honest and caring fashion. Don't forget, you could be wrong. Sorry you have to deal with this. Good luck and i hope this helps.
Prof. counselor. That sounds very hard. I would find a professional counselor to help sort through all the mixed feelings and emotions. A good counselor can help you organize your thoughts and help you put together a game plan to deal with the tough decisions ahead.