I don't know exactly, I am not trying to be rude but maybe your girlfriend isn't that good looking or maybe something about her doesn't really interest you. I would suggest that you try watching your favorite type of porn, for example if you like girls with big breasts or big butts, or maybe a different race like asian or black. My point is watch your favorite type that would make you wanna do it with girl or girls so bad, and see how quickly you would get a boner, if you get it quickly then maybe the problem is that your girlfriend isn't exactly your type but this is very unlikely.
My best guess would be that you're too nervous to do it with your girlfriend or maybe too shy, so next time you try to do it just try to relax and know that your girlfriend isn't a stranger. I can't get a boner when I see a porn scene and somebody else is in the room, I would if it was a girl I would have sex with, but if its a friend or somebody else then i wont.
I used to be a really horny person.
Last Tuesday my girlfriend said to me (on friday we can have sex) it would have been my first time.
Since then I have not been horny much and it takes 5 times the effort to get a boner.
I was sat there with my girlfriend with nothing on... and I felt nothing.
(I can get erect but it takes a lot of effort which is not normal and this has never happened before)
Also I am 17 years old.
It definitely sounds like you were completely psyched out and nervous. Don't make sex a planned thing! Stop. Relax. Forget about sex for now. It doesn't matter. Go back to hanging out with your girlfriend. Have some alone time, watch a movie, make out. Let nature take its course! I assure you that if you stop TRYING to have sex, it will happen.