I've been having sex with my girl for a while, we used to have sex for like 45 minutes straight, and I was hard as a rock! Now I can't keep it up at all, right before I penetrate, I'm an 18 year old male, what should I do? Its frustrating and annoying!

Do you have any idea what's causing this? Are you taking any depression medications or anything else? Or are you stressed? Lots of things can cause the loss of an erection. My boyfriend often has the same problem. For him, it's a side effect of his medication and fatigue/stress.

Why not try some relaxation exercises? Just relax. The more you stress, the harder it will be for you to focus and enjoy sex! If that doesn't help, see your doc.

Well, first of all, lets address her, not you.

Is she getting to orgasm? If yes, how long does it take? Can you get her off orally? Can she get herself off manually?

How fast can you get off if you don't hold back? Are you holding back during sex? Why are you holding back? To get her to orgasm? Just to draw it out?

Why is longer than 45 minutes important to you? Because you could and aren't now? Because you "need" that time to get her, or you, to the big O?

Are you her first? Is she your first? If no, how many other lovers? How old is she?

More info please... this is an adult board. The more you tell the more we can thing about what might be going on...

I certainly have some ideas, but would like to know more about your situation first.

Forty-five minutes is Way too long. You are setting yourself up to be impotent by trying to do that every time.
First, let's see, do you have medical problems? Are you stressed out? Are you depressed?? LOTS of things can cause you to lose erection.
Tell us what's going on, so we can help you.