Why am I never horny..like not even close.

I’m 22 years old, not on any birth control or anti depressants that could be causing it, I used to have a great sex drive but for the past few years it’s like not only do I not crave sex but when my s/o wants to have sex it’s like a chore. I don’t even masterbate anymore or have the desire to and I used to ALLL THE TIME. He always wants to go down on me (which he’s very good at and I used to demand basically on the daily) but I don’t even want that. This wouldn’t be a problem except for the fact that I do have a man who of corse wants sex more often than basically once a month. We’re both so young so this shouldn’t be happening & he’s been SO patient with me but me rejecting his advances all the time is driving a wedge between us (understandably). Help ladies!! Need some suggestions & advice.