Is my girlfriend's cat ever going to like me?

I want to preface that I love cats, and I come from a big family that at any given time had about 8 pets roaming around the house since I was a child. I'd like to say I know animals pretty well.

My girlfriends cat hates me. We just moved in together and its only been about 5 months, but it just isn't getting any better. The cat has attachment issues and every time my girlfriend leaves the house it just sits by the window waiting, and is generally scared of new people.

I brush the cat & feed the cat, but even after my attempts its still moot. It barely lets me pet her when my girlfriend is around.

I used to say that even cats like my girlfriends would warm up to me instantaneously but that ship has sailed. And honestly, I hate to say it, but there are tiny inklings of resentment building and I obviously don't want that to happen, but, out of all the cats I've had, this one is not the best.

Has anyone had something similar? Any solutions?