Sorry dude, I have no advice for you. I just wanted to say that I wish I had your problem...
Okay I have a little bit of advice, I haven't been in your exact situation but it's something I'm thinking about actually - how to be able to "do it" as often as you want.
Firstly, does she mind so much if you orgasm? I think that's the hardest part - each successive time is twice as difficult for me unless circumstances are exceptional.
If not, then are you able to keep the erection? I find I can manage for a while - provided I'm keeping fit and not under huge amounts of tiredness from several nights of very little sleep. Once upon a time it used to be a problem for me because I was nervous, but then I set to work trying to fix this for myself, and basically conditioned myself by masturbating in less than perfectly comfortable circumstances - eg, sitting upright in a chair rather than laying down in a comfy bed, and doing it EVERY EVENING without exception, whether I was in the mood or not, just like exercise.
Of course if I'm flat out exhausted, that's a problem too, but as far as I know, most of the time girls like it slow, and they only like it hard and fast once in a while. You should be able to get her off having sex with not too much exertion.
This is partially based on experience and partially theoretical projection... I haven't been in your exact same situation but I sure as hell would like to test myself like that!
