Caught in the Moment During My Engagement

I've been engaged to Sarah for two years now. We met at a fitness studio where I work as a personal trainer and have always had a great relationship, but lately, there's been this undercurrent of tension between us that I just couldn't ignore anymore. She is an amazing partner who works hard on her career, yet we both find ourselves busy with our separate lives and responsibilities. Despite my love for her, sometimes the monotony gets to me, and I feel a lack of excitement in our routine. Recently, there was this guy at the gym named Jack. He's younger than Sarah and me, early thirties, but he has this infectious energy that makes you want to let go of everything and just be present in the moment. We started talking more frequently during sessions, bonding over fitness goals, and I couldn't help but notice how our conversations became increasingly flirtatious. One evening, after a particularly draining day at work, we crossed some lines. Jack asked me out for a drink, and despite knowing it was off-limits while engaged, the moment felt too intense to resist. We ended up at his place, drinking and talking more openly about our aspirations, dreams, and fears. The chemistry between us was undeniable, but I couldn't shake the feeling of guilt over what we were doing behind Sarah's back. As the night progressed, we became closer in ways that shouldn't have happened. It felt surreal, like being caught in a dream you can't wake from. We shared intimate moments that I had always reserved for my fiancée, and while I enjoyed it deeply, part of me was lost to this new dynamic with Jack. Now here I am, conflicted about what transpired between us. On one hand, there's Sarah, who is waiting for our wedding day in earnest; on the other, there’s Jack, a reminder of my desire for something more exciting and spontaneous. Writing this down helps me process it all, but ultimately, I know I must decide how to proceed before things spiral further out of control.