This letter tells your lover that you know he/she cheated on you and that it's over.
1234, Main Street
Boston, MA 02123
07 April, 2005
<Recipient Address Goes Here>
You may think you've fooled everyone else, but you didn't fool me. I know about you and <name> and your <what they did together>. Since you didn't even have the decency or the guts to tell me, I'll tell you: it's over.
I won't stay in a relationship with someone I can't trust. So I hope you're very happy with <name>, and I hope you're happy with what you've done to something that used to be great.
Have a nice life.
encl: <List of enclosed items goes here>