Dear [insert person's name here],
Last week/yesterday/this morning when you showed me your true colours/selfish-nature/dark side it really threw me for a loop/ pissed me off/devastated me.
There are a lot of things about you that I like/admire/respect but as I have gotten to know you better; it seems -- there are some things about you that I can not stand/stomach/tolerate.
In case I wasn't clear when we first got together; I'll reiterate where I'm coming from. I have no desire to be with someone who can't be bothered to show me respect/be cordial/give 110%. Sure, I like to have fun like everyone else -- but I'm not sure where or why or even how you confused FUN with disrespect/ignorance/callousness? Is it fun for you to treat someone in your life like garbage/dirt/an animal? Is that what gets your rocks off?
Somehow, I've accidentally given you the impression that I'm dumb/a pushover/have low standards; so I just wanted to take a moment and make you understand/see where I'm coming from/get it.
I'm not spending time with you because I have nothing else to do/no one else to be with/ no where else to go. You aren't (and never will be) that good. I'm here because you seemed like a nice person/fun guy and you also gave me the impression that you have some of the qualities/values/mannerisms that I'm looking for in a person. Some being the operative word.
So, just to clarify, I'm not here to audition for you. And, if you think that is what is happening then I'm sorry to say that you are very wrong/misinformed/retarded.
I'm not your punching bag/girl to pass the time with/rag doll. And, seeing very clearly now, that you actually welcome people like that in your life makes me view you completely differently; and leaves me with a knot in my stomach/vomit in my throat/a sudden urge to flee in the other direction.
I'm not looking for just any guy. So, if you think you can operate at your bare minimum/lowest common denominator/worst and keep me around then you're deadly mistaken. Because I will leave and never look back. And the more you keep acting like this; the easier it is for me do so -- happily.
So, if you want to continue being a jerk/prick/loser/chauvinist/ chintzy bastard then be my guest. But, I won't be sticking around. Not today. Not tomorrow. Not ever.
Sorry I couldn't say this to you in person; but you pissed me off/disgusted me/hurt me so much that I couldn't bear to be around you another minute/second/day.
The choice is yours.
[sign your name here]