Man I have the same problem, I just don't get it. I'm 18 and my erections use to be hard as HELL. Now, its just not working. When I'm about to have sex with my girlfriend, it doesn't even go up all the way, it gets aroused and bam, that's it... it goes back down. DO you think it's a medical problem?

Moved from a 2008 post so that the OP can get more relevant and up-to-date answers specifically for his situation.