Do you have a copy of "Surviving an Affair?" There's an example on page 58:
OP, I want you to know that out of respect and love for my BS and children, I have come to realize that I must never see or talk with you again. My relationship with you was a cruel indulgence that BS did not deserve. While I cannot completely repay BS for the pain I have caused him(her) I will do my best to become the husband(wife) he(she) has been missing. I care a great deal for my family and I would not want to do anything to risk their happiness. I will not make any further contact with you and I do not want you to make any contact with me. Please respect my desire to end our relationship.
Sincerely,
WS