This letter tells your lover that things aren't working out and perhaps you should stop dating exclusively.
1234, Main Street
Boston, MA 02123
07 April, 2005
<Recipient Address Goes Here>
I know what you're going to say when I suggest this, but I really mean it: maybe we should start seeing other people. I don't mean that we should never see each other again, but I do think that maybe we've been spending too much time together and maybe dating other people would do us some good.
You might think this is a terrible idea, and that I'm just trying to soften the blow for when I write you a Dear <John/Jane> letter in a little while. But that's really not the case. I just think if we saw other people, it would give us a better perspective on our relationship. I hope you believe that.
If you want to talk this over, please call me. I want you to understand my side. Until then, you have my love and my thoughts.
encl: <List of enclosed items goes here>