How to be enthusiastic about sex?

Hey guys! So my fiance and I have been together for 3 years. I love him so much. But.. sex had gotten a bit boring. I feel like it’s almost a chore. I am always too tired to have sex, or I don’t feel like it. I’m never in the mood.

Has anyone else had this happen ever? How do I get more enthusiastic about sex? I’m only 20 by the way.

Thanks ladies