I sort of understand your girlfriend's point of view; my husband naps (I don't usually) and it is a good chunk out of the center of the day but I also let my husband run his own life :-) and do what's best for him and he does the same for me. I use the time he's napping to go out on my own and get my own life/things done or as quiet time. We're retired so with one another the rest of the time so I don't really want to begrudge him that couple hours of the day. If we have something planned, he will do that instead, doesn't "have" to nap.

Tell your girlfriend you'll trade her you being in a better mood versus grouchy when you're awake, for the chance of a nap when you are feeling extra tired :-)