I suck in bed? Girls dump me after they sleep with me.?

I am in college and very inexperienced. Because of this, I can spend hours with a girl and have a great connection then when we finally have sex she makes weird excuses to never see me again. It has been a continual source of heartbreak though I don't let them know it affects me that way and has destroyed my confidence. I just don't understand how I can show a girl a good time if they won't stay long enough for me to know what I'm doing. I have been reading on the subject, but don't get laid often enough to get good. I don't even know what my question is. I am just very upset right now.