RAWR!, Friend troubles. Options

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Ok..

I know this one girl, say her name was Mary. I am the only one who ever talked to Mary and ever listened to her because everyone else hated her. She called me her "best friend." whatever. screw that --;
Ok, Mary has finally quit a sport, but she constantly comes to just do the sport by herself. I have no problem with that at all..however..I finally made the better team and I came up to her one day and said, I made it instead of this girl...named Kelly.
Well, Mary goes off saying I'm going to bring the team down and blah, blah, blah. And I tell her I know that, but I'm going to try hard not to. THEN she has the nerve to tell another girl [even though we all heard what I said that I'm not that good and stuff]..Susie in private about why the coach picked me instead of Kelly and Kelly is sooooooooooooooooooo much better.
THAT'S NOT ALL. Then, she has more nerve to tell some girl at school about what the heck was the coach thinking when she put me on the team!


RAWR! I want to kick her arse so badly. Anywhooo...What should I do? Continue to ignore her? Or...?