No libido?

So my bf and I have been together for a little over a year. We had sex quite a bit when we first got comfortable with each other, but ive never been super into having sex all the time. (My ex and I had sex maybe about once every 2 weeks) We used to have sex like 3-4x a week and now I’m lucky if im horny maybe 2x a month :/ I’ve never really been super intimate and this is only my 3rd boyfriend. I’m turning 23, I feel like my sex drive should be higher??! I feel like I dissapoint my bf cuz he wants it more than I do and I say no all the time to him :/ he doesn’t force anything on me, but I feel like my libido should be higher since Im still younger?! Any ideas why I never want to have sex?