Ok I need some serious help here...

I am 18 years old... I get turned on all the time and when I am by myself even without masterbating my penis stays hard for ages.

When I am with a girl I can be as turned on as hell but my penis not even get to any point of hardness unless she be really rough in the way she gives me head.. as soon as her mouth leaves it starts shrinking right away. i dont have time to put on a condom or even stick it in her it just goes limp...

Is there any suggestions on what I might be able to do to stop this?

The chick I'm with says she dosn't care.... but I do...

Thanks and I hope some one answers this even with a little bit more info to help me understand what I'm going through :-(