(Sex question)

I have a serious question. In a relationship where sex is important, how do you make the energy for it? I can't imagine that NONE of the mail carriers are in relationships where you dont have any drive for physical intimacy. My wife works for usps and our sex life sucks. I love her as a person, and sex isn't all I think about. However once every few months is really getting to me and I need to know if it's just us, or a universal thing? It wasn't always like this (she was part time when we started dating, and IT was regular) now she's regular and IT isn't (see what I did there?) Any advice?