Erection Problem Before Sex, And Im Not Even 20 Yet!

I am a 19 (nearly 20 year old) guy and I have been going out with my girlfriend for 3 months and find her extremely attractive. I have never been so attracted to anyone ever before, and I have never cared so much for anyone either. I have had 3 previous sexual partners, although two of these were one night stands. My gf was a virgin and said I was the one for her, but when it came down to it I couldnt get a decent enough erection. Eventually all feeling down there was gone and I was completely turned off. A few nights later we tried again and again I lost my erection just before, despite being completely stimulated beforehand. In the morning after waking up I managed to maintain and we did it for the first time, but after this the problems persist. Initially I though it was nerves, and we have managed 4 times, although the 4th time, after 30 mins or so, I completely lost my erection and we had to stop. Last night we tried again and this morning too, and each time the same thing happens. I have no idea why this is I cant remember it happening before. My gf says she doesnt care because she loves me for being me but I know eventually she will get pissed off and go find someone else, I feel really inadequate and really depressed over it now, does anyone have any idea why this is happening me??