Why can't I get it up with my girlfriend?

Here's some info to eliminate some common answers. I'm freaking 18 years old so nothing medically is wrong with me. I easily can get an erection when I'm by myself. I had no problems getting an erection with the girl before this one. This is the third time we've tried having sex and I just can't get it up. This time we took it really slow. We were probably in bed for and hour and nothing happened. I achieved an erection for a little bit but I didn't think I was fully hard enough to start going so I kept working on her and then it went down and STAYED down. I'm pretty sure my mind was clear. I was relaxed and everything. Wasn't drunk or on drugs. And this girl is HOT. She's like my dream girl. I really don't think it's her but at the same time, when I compare the girl before her who I had no problems with with this new girl, the only difference is really the girl. Although I really didn't care about the last girl. So maybe subconsciously there is something wrong. Help!