Problems with erection after ejaculation

I'm a 18 year old male who really needs help. I'm in a sexual relationship and I'm embarrassed to admit but not only do I have a limp erection and early ejaculation I cant seem to become erect again after sex (for hours). I always seem to finish early leaving my girlfriend unfinished when she tries to get me going again i just cant do it. Is there something terribly wrong? Can you help?