unable to maintain an erection during sex.

Hi im 17 years old and ive been with my girlfriend now for about 4 months, my problem started when we first started to try and have sex, she used her hand/mouth to get me hard iput a condom on and got into position etc and i just went down, not completely but not neough to have interocurse. This was a problem a couple of more times until i decided to make her do alot of foreplay on me, and it worked, it didnt last very long but it worked, then the next time i had troubles again, but managed eventually after much trying to maintain an erection and have sex for 10-15 minutes, altohguh i didnt feel i had a full erection, recently we tryed to have sex again and by the time im in position i loose my erection, its lyk if i get in there quick enough hard i can maintain it but 10-15 seconds without any sort of action i go down. I dont think its a phsyical problem though becuase jsut at college or on bus or sometihng i can ahve an erection for 30-40 minutes without anything to even trigger it.if i do get it in it goes in and slips out. During oral im fine, or even if we are kissing/feeling me up i get one. We are completely comfortable with eachotehr and talk about absolutely everything, it doesnt bother her cus if i fail i just give her an orgasm orally, i dotn get the problem, im pretty sure its not nerves i dont know, i suppose when we planned to do it and i tihnk about it im liiekk "what ifit happens again" kinda thing. I used to think it was condoms beign too tight wateva but she is on the pill now and i still had the same problem. can anyone help, would be greatly appreciated:)