A Secret Meeting Before the Wedding

I'm engaged to my boyfriend of three years, and we're planning our wedding. Everything seems perfect on the surface. However, last week, I slipped away from a family gathering for about an hour. I met up with John, an old friend I've always had feelings for but never acted upon until now. It wasn't planned, it just happened when we ran into each other at my sister's house. We ended up in his car on the side of the road, and before long, we were kissing passionately. It felt like something out of a movie, spontaneous, intense, and full of electricity that I never knew existed with anyone but my fiancé. But this was more than just a momentary lapse; it was about longing for what could have been. We didn't go all the way, but there's no denying we both wanted to. As soon as I left, I felt a mix of guilt and confusion. Was this just physical attraction? Did my feelings stem from unfulfilled fantasies or true deeper emotions? This one-off encounter is weighing heavily on me, especially since John is married with kids. It complicates everything yet makes sense in another way, but how can I reconcile this with the life I'm about to build with someone else?