Dear …
I want to apologize to you for all the trouble that I have caused your family. I understand that my indiscretions have caused __(briefly describe what has happened)__. I am truly sorry for this.
It was wrong for me to allow myself to get involved with __(Name)__. I know that now. At the time, all I could see were stars in my eyes. I was blinded by the love I felt for __(him/her)__ and was not able to think right. Now the mixed emotions of my heart have swung from love and joy, to hurt and remorse.
If I could turn back the pages of time, I would do that. It is with deep remorse that I look upon all the grief my infidelities have caused your family. You must all hate me for what has happened and I don’t blame you at all. Out of a forbidden love I carried for __(Name)__, I unknowingly put members of __(his/her)__ family at risk and I take full responsibility for what has happened.
I never meant to cause all this trouble in your lives and I am genuinely sorry for this terrible mistake I made. Perhaps one day you will find it in your hearts to forgive me for all the heartache I have caused. Perhaps one day I will be able to forgive myself.
Sincerely,