What's your personal opinion on this situation?

[Love you.]

I just need supportive comments or something because I just can't get this issue out of my head.. I'm 15 and I've dated this guy for TWO YEARS. We've recently broken up and to be honest.. I still love him to death :l . But I know he's over me.. but that's not the problem. The problem that makes everything worse is that my newly ex-"friend" is now going out with him.. for the SECOND time since me and my ex broke up. The first time he dumped her for another girl and me and her made up.. I found out that before he went back to her that she said if I ever went back out with him she'd be pissed off. But when I told her sister that I was upset, she said I needed to "F*cking get over it." Honestly, she only cares about herself and doesn't give a sh*t about anyone else. She thinks everything revolves around her and that she HAS to be right even when she isn't. I went into a depression before this happened (after the whole break-up) and she always said I was pathetic for it and she had no sympathy. BUT.. the first time he left her (which was like a 2 week relationship) she said she was depressed ?! (WTF?) I can't stop their relationship.. But shouldn't she have had the respect and dignity to back off from him? I don't know what to do.. I was almost over him until this happened. I'm best friends with her sister so I'm always around them.. I can't take it anymore. I can't even sleep because of it.. this is not what I need to go through right now. Most of this was venting and getting things out, sorry for the length.. Any help )': ?