lol @ hormone levels etc. being the problem.

So you say that after sleeping for a bit and trying again it works out?

I can't say what's going on definitively; I'm no doctor or specialist. But it wouldn't surprise me if it were purely a psychological problem. If it is I can't give you any advice, but I wouldn't jump to conclusions that you have some physiological problem if I were you. It sounds like everything is functioning properly down there.

I think that Hollywood and porn have given us harmfully incorrect depictions of what sex should be like for most normal men. It isn't as automatic as they make it out to be. In fact, male pornstars are those few and far between freaks that can keep it up no matter what. Normal men aren't like that.

Do you think you could communicate to your partner how you feel about this? As you said, it doesn't bother her, but if it bothers you you should either simply live with it the way it is IF you can find sex to be able to be fulfilling for you OR you should explore doing things differently if you think it will help.

EDIT: I see that a lot if not all of my advice had already been dispensed, but just to... uh... reinforce it...