kangaeru is right if you cum too fast - masturbation will curve that, especially if you masturbate earlier during the day...
Personally, I've found that practice is the best way to build stamina. I used to get tired real fast but now I amaze myself once in a while after looking at the clock... and once and a while i'll take a quick sip of water that I keep by the bed anyway.
Tell her you need her to help you practice and that the more you do it the better you'll get - not only is that probably true but thats a great excuse to f* every chance you get
