advice please!! 19 years old, experiencing some ED...

I am a 19 male, good health, experiencing some ED. i never used to have this problem. it happened for the first time about a year n half ago.. ever since then, i have had troubles maintaining my erection. With my new gf getting very stressed about about it too, i am desperate to seek help or some type of advice. I feel like a loser. I used to be able to maintain one for an hour or more with intercourse without a problem.. even sex more than once a day.. Then bam! out of nowhere.. an almost instant change happened with a woman that i was talking to at the time. Now i dont know if im too worried about it happening again or what but it is very stressful. I can maintain an erection just fine when im alone and right when we start foreplay, then when we get ready to have intercourse, it suddenly goes dead. I could be worrying too much maybe? On the occasion i can get it up long enough, but i have to work so hard at it, almost to the point of ejaculation, that i dont last long at all. it has happened with 3 women now... i dnt know whats going on.. please help me... ive tried about everything i know to do.. i really wanna get this problem fixed to please the woman i care about.. is it something im doing? is it in my head? i am very attracted to this girl... and have a high desire for it.. i just dont know whats going on.. Any thoughts, advice, or suggestions on what could be happening.. would be SO helpful...Thanks!