Dating as an adult

Okay, so this might be weird and maybe I'm alone in this. But I'm almost 21 and my longest relationship lasted only 5 months. And that was when I was 15. Ever since I became sexually active, the guys I've been dating have never stuck around very long. And now that I'm a grown woman in college and all that, I haven't dated anyone in the last year. I've hooked up with a couple guys, but every one I talk to doesn't seem to be interested in anything more than a hookup. Anyone have any tips or ideas as to why I have such terrible relationships?