Meh,
I went down on my last girlfriend, but she didn't like the idea of blowjobs and I didn't mind one bit.
My point being is, are you just going down on her in the hope that she'll return the favour?
If she makes you happy, don't let a silly thing such as your girlfriend sticking your trousser saussage in her mouth the main thing in your relationship.
Blowjobs aren't actually that great (okay, admitedly I'm not an expert), there's a lot of hype about them but: They're not that great and certainly not important.
Doesn't your relationship have anything in it beyond foreplay?