How should I do ?

This isn't about a crush or anything but I really need help here. So there is this girl who I'm friend with since the beginning of high school (last year), and we started hanging out with each other because we were both friendless and she was nice to me. And I made new friends during the year so she just hangs out with us. But now I guess she's starting to show her real face ? Like she's not so shy anymore, she's even mean with most of the people at school, and I asked around and found out that nobody likes her they just talk to her because she's my friend. But I don't want to be friends with her anymore, we both changed and her personality doesn't matches mine like it used to. But I don't know how to "break up" with her, and I'm feeling super guilty because she doesn't have any other friend so she will be completely alone, at lunch, in class, during breaks. And I don't wanna be fake by still hanging out with her even though I don't like her anymore. So what should I do about that ?