You need to stop worrying, tensing or whatever it is your are doing that creates an issue about this. Oral sex should feel good, whether or not it's able to bring you to orgasm.
I'll say this though, it's very much a matter of technique. The mouth is not so different from a vagina or a pair of lubed up hands. It's all about how it's done.
When my partner goes down on me, I can tell her to do things that make it last a long time. Which is great for just enjoying the ride and feeling good. Or I can tell her to do things I know bring me to climax a lot faster, because they're more stimulating.
It's possible your girlfriend doesn't have the proper technique or that you two aren't in sync. It's hard to guess at.
It sounds like you're not aroused by oral at all. That's a pity I think but we're all different
You should be able to learn to at least relax and enjoy the feelings.
Can your GF get you off with a hand job? If so, she could do that and gradually incorporate her mouth into the play. Or you could have regular sex where you break off and she does oral for a minute and then back again.
That way you can learn to associate the feelings with other positive experiences and perhaps come to enjoy them more.