I think this guy sounds like a jerk and you are better off without him. Try to stay busy, go out with friends, whatever it takes to keep your mind off of him. As far as your 'best friend' dating him, I think a true friend wouldn't date your ex, at least not this soon and not without asking you first. Hang out with your other friends, because it doesn't sound like she has your best interest at heart. Chances are if he was a jerk to you, he'll probably be a jerk to her. She should know better, but some people have to learn the hard way.
I hope you feel happier soon!
