I am having a fight with one of my friends at school?

There is this girl who originally invited me into the group when I had no friends. She was kind and nice but when stuff started to go my way she became really mean. e.g. I started coming top of my classes ( she is in all of my classes), I got free plane tickets to places, started taking really good photos, coming well in cross country and so on. One day she started becoming really mean and taking things that were important to me, and talking about them like they mean nothing. Plus when I say something that is a bit of a shock, like when I won something in a raffle, she went silent and then 15Min's later she started asking me questions and basically telling me what I should do with it and why I don't deserve it. She has done that a few times now. She also ALWAYS and I men ALWAYS has to have the first go at everything unless it is dangerous. When it becomes not dangerous she will take over. And sometimes the only reason she hangs around me is to get food. I have tried talking to her but she kept bringing up random things like her favourite food and ice cream. And the worst bit is that if I decide to leave the group and she turns all of my friends against me, then I will have no one to play with as no body else hangs around me. I am confused and don't know what to do. Can somebody help?