My friend says she has outgrown me. Why would she dump me?

She now hangs around with the really popular people and has started dating guys at school and changed her image, wearing cool clothes and ditching her gles. She says she is sick of me and is now too good for me and her new friends are so much better. I feel hurt. I say hello to her in the corridors and she ignores me. The popular kids won't let me be a part of their group. I have tried to make it up with her. I have grown up and realised I don't need friends like her. They are making my life hell though, spreading lies about me and saying I am like a big dog.