Mismatched Sex Drives

I've been with my wonderful gf for nearly 4 years. We've generally had a really well matched sex life, same labido, same kinks ect. But for the last 6 months or so my girlfriend's sex drive seems to have completely disappeared. Nothing has happened or changed that I can think of, we seem closer than ever and she's super affectionate day to day. But seemingly has no desire for sex anymore.

We've had a few conversations about it and she can't really give me a reason why, or she'll say she does want to, but doesn't act on it.

Obviously I've not put any pressure on her, have reassured her ect. But it's really starting to get me down, I just don't understand what's changed and am wondering if this is just a *forever* thing now.

As I've said, I think we're closer in every other regard than we've ever been, and she seems really happy. But my drive is really quite high and I'm starting to feel almost resentful towards her, which I feel terrible for.

Any advice?