Why don't I like myself and things about me? How can I change that?

I used to be picked on to the point where I have ZERO self confidence. When people say I'm pretty, or compliment me in a good way...I never believe it. I say thanks and this and that, but when I know them well...I keep asking for reurance. "Do you really? Why? You're joking right??" type of stuff. I look at myself in the mirror and still say "ewww" It's not just about being pretty...it's like I don't like the stuff I do or say...or just things about me. I LOVE IT...and at the time enjoy things, but then I get a glance in the mirror and think how stupid and useless I am. how can I stop this? I want to feel good about myself because I should. I'm not perfect, but I'm a good person and should be happy about my stuff in every form. PLEASE HELP.