Why have i not had a real boyfriend yet?

I'm 15...(and just to describe myself) I am mixed ethnicities, slender/fit, 5'10", and I am a model in Hawaii. yet, I have never actually had a relationship yet.. yes, i have had my first kiss though, but thats not what i am worried about. I feel trapped whenever i like a guy and the guy likes me back.. i seem to like it better when i'm the one liking the guy, without him liking me back. As soon as i find out that he likes me back, i just start to push away, i don't know why i do , but i do. I want to stop doing that.. but I don't know how.. and plus I am shy and kind of quiet before you get to know me.. I NEED HELP ! how can i get over this trapped feeling???