I think openyoureyes was saying to masturbate before the session starts, so you're running on empty when you start, and I think that's the simplest solution (though you still may be able to orgasm even without anything in you to ejaculate).

You could try cock rings, they can help you hold out longer and possibly your partner will be able to cum first.

But I would recommend training and practice the most; whenever you feel yourself about to get close, stop and switch to some other activity (one you preferably don't find arousing), until you're able to control yourself again. It takes practice to build up to where you want to be (with anything, really), and if she's patient with you, you should be able to hold out for longer and longer periods.

You can also try getting her close to orgasm in some other way first, so she's able to orgasm very quickly, long before you.