...I Just Couldn't Resist...

So, I'm in my mid-twenties, engaged to the most amazing woman who's just as committed and excited about our future together. We've been talking marriage and kids for over a year now...but one night last week, it all felt different. We were at her place, everything was quiet and cozy, just watching some movie she really liked. I mean, we usually do that, but this time something was different...there was this tension in the air...and somehow the thought of cheating crossed my mind out of nowhere...I know it sounds crazy, right? Anyway, as we were cuddling a little later after the movie ended, there was just this pull I couldn't ignore. It's like...there's this other side to me that wanted something else... So here's what happened...and it's all so confusing. After some sweet but innocent kisses, my hands started wandering down her shirt, and she gently pulled back, asking if everything was okay with us... But instead of answering or telling the truth, I hesitated...and then, without another word, I whispered something along the lines of 'not right now' as I kissed her neck. And then I just lost it...pushing her into her bedroom and taking control... Oh god...it was so explicit, so raw, so different from us. My hand grabbed her thigh hard while my other hand moved inside her underwear without hesitation...and she didn't stop me or push back at all, just stared in shock and then leaned in to kiss me deeply as I fingered her... ...I can barely even admit it to myself how much pleasure I felt knowing that this was outside our norm...she gasped softly when my fingers went deeper inside her... It didn't last long but the way she responded, like she wanted more of what I was giving her...it just made everything worse. Afterward, I pulled back and it hit me how wrong it all felt. I told her it was a mistake right after we finished. We haven't talked about it since... So yeah, that's why I'm confessing here instead of being honest with my fiancée first...I just don't know what to do or say now.