I feel it's important to find a counselor that specializes in dealing with infidelity. They need to know the dynamics behind all the heavy emotions that you both are carrying.
This article may help
I know our therapist spent the first couple of sessions getting to know us and our marriage prior to getting into all the infidelity issues. She wanted to know *us* and our marriage to help better understand and to give us the proper tools and exercises "Don't give up, the beginning is always the hardest." My Mom:)
My tolerance for stupid shit is getting less and less.