Sex drive

My fiancé and I have been together for almost 5 years and I feel like my sex drive is completely gone. We usually have sex once a week but that's not always guaranteed. He says that he doesn't try to initiate it more because he knows that I would just do it to make him happy, not because I really wanted to. I'm only 24 and feel like I should be wanting to have sex all the time! Is this normal, does anyone else have this problem?