Relationships go through different phases. Sex can be variable for a lot of different reasons. Have you thought of trying to add some newness? like trying out a fantasy setting - taking a trip somewhere special, kind of thing so that you are out of your routine?
During the childmaking phase of our marriage my husband's and my sex life was in the toilet, but it is much recovered now!
So whatever may be going on between the two of you, do not give up on your relationship if the only thing not working right now really is the sex drive for each other.