I am an 18 yr old guy in good physical shape with no medical disorders or conditions that i know of. I have been having regular sex since i was about 14, all with the same girl as i was in a long term (4yr) relationship. I had never had any erection problems before, infact quite the opposite. Ive always had a great sex life, done pretty much everything with no problems.
Well last night, for the first time, i had difficulty getting an erection. It was my first time with a girl other than my girlfriend. To top that off it was cold, i had quite a bit to drink and i think i was nervous/guilty.
No matter what i did i could not get it up, no matter how much i tried or she tried. when i did finaly get an erection about 30 mins later, i put a condom on and began "intercourse", only it was very different from what i was used to, it was very tight, it did not feel like it was moving and she sounded like she was in pain. This resulted in me losing my erection no matter how hard i tried the rest of the night.
I was appaled at myself and very embarrased. My friends said that i have nothing to worry about and its all in the head, funny thing is even now i cant seem to get an erection when i try, i did once today while i was watching television but once i noticed it began to leave and did.

Is there something wrong with me ? is it all in the head ? was it the conditions that let me down ? should i be considering medical help ???

thank you for your replies