My guess is that the counselor will tell you that there's not much that can be done while your wife is involved with another person. That has been my experience. I wouldn't expect to get anything out of the session except to let the counselor know what's going on. You won't get any relationship work done because she's too busy thinking about the other relationship.
As for where you go from here...that depends on your convictions and your temperment. You'll find a wide spectrum of coping strategies here, each one suited to different values and priorities.
At the very least, you should consult a lawyer (one who specializes in fathers' rights if you can find one) to educate yourself about your rights and responsibilities. Channel as much of your energy as possible into being the best father you can. They need you to be the stable, focused and rational parent that your wife is not.
