How do I make someone feel worthless?

First of all, how old are you? If she's making your life crappy it's most likely because she is jealous of you. Usually girls don't like each other because the other one has something they want. Either material or psychical. If I were you I'd go up to and ask what her problem is. Big the bigger person, by you trying to make her feel like **** it just shows you're just as petty as she is. I know it's hard to ignore but seriously try. If you don't give her a response she will eventually leave you alone.