engaged and freaking out

I've been in a relationship with the same guy for the past 5 years, we have lived together for 3 years, we have been friends for 6. I'm afraid I have never really been physically attracted to him. Lately it has been even worse I don't feel attracted to him at all and I feel terrible because I get butterflies in my stomach when I see other guys, one of his friends mostly. My fiancé is so nice, kind, smart, talented, creative, I don't know what to do. I'm 24 years old, starting my teaching career, and taking Prozac for depression. Any advice is super appreciated!