I've been dating my girlfriend for almost 3 years now. Everything about our relationship is great, she's a real catch. Unfortunately, she has a lower sex drive than I do. This is nothing new, since the beginning she's always been more passive and less sexual than me. She really enjoys sex once we get started, and rarely turns me down, but I have to put a lot of work into getting her in the mood. Recently, I'm getting tired of having to always be the one that initiates things. It makes me feel like she doesn't physically desire me. I've been getting sexually frustrated and often masturbate secretly to try to quell my rampant sex drive.
Does she know that you masturbate? You said you do it in secret - maybe if you let her know that you do then she will look at things differently. Perhaps she can help with that even if she is not in the mood for sex herself, I guess it depends though. But it's possible that she would enjoy pleasuring you that way and it would keep both of you satisfied. I don't know how you would suggest that directly though, but you should let her know that you are masturbating if she doesn't already. Gague her reaction and act accordingly.
I think most girls have lower sex drive than their boyfriends. My girlfriend certainly does, though not to the extent that your does. If this is a very serious problem, but you want to continue the relationship, I would recommend some kind of couple's therapy.
Women have lower sex drives than men do. This is normal for humans and what you are describing is also normal.
If women had the same sex drive men did, no one would ever stop fucking and we'd all starve to death.