Can't remember the book
So the main character is a girl in high school and she talks about her friend from elementary and she would play with her but her parents and the main character moved. The main character is being bullied at school and lives the popular girls boyfriend but then the old best friend shows up and says her dads abusing her so she hides her in her room. The bullies start dying and one day she goes to her tree house and there's a body in it and wrestles the friend and realizes it wAs her and by this time she's dating the boy and he comes and she yells catch her and there was no one. She killed the people and didn't know because the friend was her second personality.