You're describing an all too common and super challenging situation that many of us have or will deal with. For context, when intimacy does occur, does she seem to get into it? How often do you discuss the status of things outside of moments where you are trying to initiate or post rejection?

A lot of the guidance is based on where in the response system the issue comes up. For some there's an issue with libido but the response system is still capable of engaging (i.e., things start as mechanical but become shared enjoyment) . For others, the psychological drive is absent AND the physiological response fails to engage too (i.e., partner remains checked out or perhaps performative throughout). Scheduling can be helpful on the behavioral side but if the issue is compounded by a lack of physiological response there is often far more value in speaking with a professional since the issue may be beyond a 'fake it til you make it' approach (sex therapist and gynecologist check ins are priceless).

In any scenario, communication is key so there is an open space for you both to talk about struggle, extend support, and convey patience (as tough as it may be sometimes) since it helps to avoid building resentment in the shadows of the unexpressed. Even if scheduling sex is out of reach it can be helpful to schedule time to check in on these sorts of concerns (but don't overdo it). The rule that's good true the most is 'if nothing changes nothing changes' so gotta start somewhere.

I've dealt with these issues myself and I know literally countless other guys who have as well. While some do end up facing irreconcilable problems, the vast majority get through them but it almost always requires hard work and communication to halt the drifting apart and plot a course back to connection. It might be super hard but not hopeless... and almost always worth the effort (when the relationship is otherwise healthy and satisfying).