This letter tells your lover that you want to break up because you've met someone you would rather be with.
1234, Main Street
Boston, MA 02123
07 April, 2005
<Recipient Address Goes Here>
I don't know how else to say this, so I'm just going to say it: I've met someone else. I didn't intend for this to happen, but it just did. <circumstances>, and I just can't stop thinking about <him/her>. I thought I should be honest with you rather than try to conceal how I feel.
You know how much you mean to me, and how much we have shared over the past <time>. I think it's best to accept the fact that we don't work as well as we used to, and that it's time to move on.
If you want to talk or just get mad at me, please give me a call. I just don't want you to hate me. Remember that you have my love and my thanks for being so wonderful.
encl: <List of enclosed items goes here>