Sorry for what you're going through. You are hardly alone. I think it is an inevitable pattern for all relationships to hit a cooling stage. Heck, I'd imagine that even the singer Sting and his wife Trudie have limited their sex sessions to being under two hours nowadays.
There is no one perfect answer to your dilemma, as each relationship is like a person and is unique.
Nonetheless, some random thoughts about it all from me: Although we live in a day and age where women wish to be viewed equally to men (and for things like salaries, they
should be), I do not believe they are identical to men in their thinking, especially in sex. Women's sexual problems are usually outward manifestations of non-sexual problems. Within themselves, it could be a battle with self-esteem or shame or financial worries or something else. In terms of their partner, i.e. YOU, it could be either (1) she doesn't feel understood or known (2) she doesn't feel appreciated (3) she wants attention in her "love language" (that shitty relationship task where you have to figure out what is important to her, whether it is time or gifts or something else.
If you are patient, time can change things for the better with her.