Only you can figure out why you might feel this way? Does your fiance know you were involved with this other guy before you knew him? How does your fiance feel about the guy? Are they friendly? Competitive? Like/Dislike one another?

If they get along well, could be you're jealous and want to hurt your fiance for having a successful relationship without you. Does the other guy have a girlfriend/partner now? Could be you don't like her?

I don't know what has been going on with your relationship with your fiance. Have you disagreed (doesn't matter how amicably) about something that you unconsciously think the other guy would agree with you on? Is the "honeymoon" period over so the exciting chemical feeling of being in love has worn off (part of what gets us to pair in the first place, the chemically-induced feelings but they don't last) and you miss the excitement/high of talking all night and constantly being absorbed in the other and knowing they're absorbed in you?