Im 23 years old and I have no problem building an erection for sex. The problem for me is that when I exert lots of energy (usually to get my GF to finish), my penis goes limp. It's as if my body is recovering and wont let blood in my penis. This has NEVER happened to me before... Im trying to figure out what could be the cause. During regular intensity, I can keep it up, but when it gets rough, it goes down. This is when I need it the most!

I exercise regularly but dont do much cardio. Could that be the problem?

Please help...