What do you think about my break-up letter? Should I send it?

Breaking up by letters, texting or chat-rooms is very embaring and you should want to be the better, mature person. I've only broken up in person and usually I've been pretty good friends with the girls I've gone around with after. You should rephrase your letter so it can be a warning letter, not a breakup letter. That way if your 'open relationship' doesn't become a proper relationship you can tell that it isn't worth it and this guy your talking about will be fooling around with randoms preeety much the rest of his life.