I dont know what to do

I love my fiancé but our sex life is not very good for me. We’ve been together for 5 years and I don’t want to leave him but I’ve tried to communicate to him that it needs to change. All he says whenever I say it needs to change is “it will but what happens if it doesn’t” all I say back is “it just needs to change”. We argue everytime he asks for it because I tell him I don’t want to do it. He begs and begs until I finally break down and give it up. We usually end up fighting about it because he constantly wants to do it but I space out when we do. The only thing I think about is all the stuff I could be doing instead. What do I do? Please be kind.♥️