May have little to do with physical endurance and more with mental stressors... you might be "in a rut", so to speak.
Sure, it seems better and all, but maybe if you tried spicing things up a bit you'd find the LOTS and LOTS of other ways to give pleasure- for both you and her- that aren't just pounding the organs together.
Change up the positions, try some new things- go together to an adult store and shop for sex novelties. You never know how much fun you can have until you're having it.
