Sometimes I cry at how worthless and ugly I am?

In all honesty, I get called pretty a lot and complimented a lot, but I can never seem to believe them. I just go "thanks" when someone compliments me, but I honestly don't understand how anyone can say that about me, when I hate myself. And there's even times when I'll look into the mirror and cry at how stupid and worthless and unwanted I am, and then how every other girl is better than me, through looks and personality. Like when I meet someone for the first time, I try to come across as overly confident, or overly funny, to stand out and be memorable. I'm 17. Is it normal to hate myself this much, and if not, how can I help myself? Please, I'm desperate for help.