Dear friend, ex-friend now, I guess:

I don't know what sparked that last bout of unprovoked nastiness - jealousy, paranoia, a bad day at work or a sore back. You were behaving like a furious, unbalanced teenager. Maybe you were trying to trigger some kind of reaction from me, I don't know. I don't really care now either. All I want to do is get it established that no, I did not lie to you. I had no reason to, you had no reason to suspect me to, and just because your version of things doesn't jive with my version of things does not "prove" that I lied. This is real life, where it's not always easy to establish exactly what happened, it's not a spy story where the slightest discrepancy is suspicious and means that the other person is in fact your enemy.

What is killing me is how nasty you turned. Arguments are one thing, but you really want it to be me being a liar. I won't be able to forget that. At some point you may apologize. But I really can't trust you again. I wish I could because I really like you and you're a lot of fun at your best.

I don't if I'll be able to really "break up" properly. Maybe we'll just drift apart and not talk much after a while. Just a bit apathetic now. I'll go hug my cat again.