Can my relationship be saved?

I’ve been with my fiance for almost 4 years now. For the most part we have a great relationship. He treats me wonderful. He’s the nicest, most caring man I’ve ever been with. There is no question he would do anything to keep me happy. The problem I’m having is being sexually attracted to him. I think he’s a good looking guy. He has a pretty good body. But when it comes to having sex, I don’t feel the passion. My sex drive seemed to decrease over the years. Is this normal in every relationship after being with the same person for so long? How can I overcome this? I don’t want a great relationship to get ruined because I can’t get turned on anymore. And I certainly don’t want to start getting the urge to cheat. Help!