I've foud an interesting page on the Net. It raises a very important issue:
I've had an affair, how do I tell my partner?
I'd give some hypothetical advice, but it depends where are you from.
If you are from a mediterranean country: shut up! God will forgive you in the Paradise, maybe. But if you are from a Protestant country, you must tell all to your partner because you must get the bumps during your life and not in an unknowing place.