confession of a married artist

so my fiancé and i have been together for five years now. we were just engaged last month but the thing is there was this moment about two years into our relationship when i had to admit something to her. i started noticing another woman, in my art class actually. she's stunning, talented too. her name’s lily and we hit it off right away as friends. turns out she’s also single. nothing happened between us for months but then one night, after a few drinks at the studio, things got hot and heavy between me and lily. i told her about my fiancé immediately after and that it was just once, a mistake but… it felt so good. we haven’t spoken much since then but the other day while walking home from an art show, lily sends me this flirtatious text. i couldn’t resist replying. turns out she’s single now and misses our time together. i told her about my fiancé again but somehow things progressed quickly. ended up seeing each other last night. it was so intense but… i know this can’t go on, right? what kind of person am i? still haven’t told my fiancé though. just trying to sort through everything for now.