My EX girlfriend is stalking me?

She has been following me everywhere i go and i don't feel safe. She is insane and that is why i broke up with her she would freak out about nothing and attack my friends sister or even me. She is a very violent person even during sex which i didn't like. She has sent me creepy texts telling me if she can't have no one else will. She said we are like Romeo and Juliet although she only one of us can live. What should i do?