I'll be blunt. Don't get married. Wait until you understand what you want better. You are very much at risk of talking yourself into getting married and regretting it a few years from now. Your ambivalence is meaningful. Listen to it. By delaying marriage you run the risk of losing your fiancé, but that seems like an appropriate risk in this situation.

Maybe you're polyamorous. I recommend reading about that relationship orientation. People here seem to be against the concept of polyamory, but I'll tell you it's a completely legitimate thing and some people finally find their way to happiness by accepting it.