My mother says I'm worthless?

I'm so sorry for you. This is emotional abuse, plain and simple. Your mother is certainly under stress--all parents are--but she is not emotionally mature enough to cope appropriately. Has she ever become physically abusive towards you? Would you consider living with your father? I implore you to tell an adult, a parent of a friend, a teacher, etc... Your mother needs help and you need to be safe! Please update...