Why do I want to destroy myself?

Hi, i seem to have many different problems yet I have such a good upbringing that I just don't understand why I'm like this. How can i one day be manically happy and the other extremely empty, and feel like i'm just a shell? Why do I cut myself and scratch and bang my wrist? Why can't I sleep for days on end? Why do i have a strange relationship with food and why is it that I constantly make impulsive decisions which ruin things in my life? I don't understand myself at all, how can one person have all these different things yet appear to everyone but my friends to be sane? i'm so confused and I've never spoken to anyone before, I know I need help.

emma i think you know whats wrong,but your scared you have to get help it will only get worse without it.