When some individuals rebound, they become so enthralled with their new, problem-free relationship that they refuse to look at the reasons why their 'old' relationship failed. Your counselor should be able to help your husband to understand the importance of leaving his new relationship behind if he is interested in attending marriage counseling with you. However, if he refuses to end the other relationship, I would ask you to consider staying in counseling, for and by yourself, so that you can have personal support as you try to deal with the problems in your marriage and their impact on you. You cannot 'repair' this situation on your own and so your husband's decision about this other relationship will be crucial. Time is another important consideration with reference to your situation in that if you and your husband do attend counseling together, you must allow adequate time for the counseling process to have a positive effect on your relationship.
Time is also an important factor with reference to individual counseling, as well.
Many individuals and couples begin counseling with the idea that they should see immediate results in the sense that their problems should begin to diminish in a short amount of time. Some therapists spend a significant amount of time trying to gain an adequate personal or marital history and then proceed to direct their attention toward the individual's or couple's most significant problems. Other therapists spend less time concentrating on past history and move rather quickly toward addressing an individual's or couple's problems. Either way, the counseling process itself requires time and it also takes time for the results of that process to be recognized. Counseling is a very difficult process in that one confronts and discusses problems that could not adequately be addressed without a therapist's help.