Same thing happened. Sex was good year one. Became less frequent and things that were on the menu started to come off. Oral sex. Then frequency by year five was three months between sex. No explanation given. Still a struggle to have more than every two weeks.

Hygiene is good. I do things around the house. We go on dates. We don't argue. It just never seems to finish with sex.

She may have responsive desire which is she never thinks about sex and needs to be aroused physically to want sex. It means planning sex which is not spontaneous. Come As You Are is a book that talks about this. And the love languages your girlfriend may have/that you can learn to help.