Consider going to a marriage counselor, if you were planning to get married, try pre-marriage counseling. If you really love him, and if he really loves you, you’ll find a way. He most likely will agree to go to a marriage counselor.
Try and get him to confess to any other acts of infidelity before you go to counseling, you should know that for your relationship to work, you will need to trust him again.
Hope everything works out,