i cheated on my fiancee...

my name is lucas, im 25 and im trans. ive been engaged to rachel for the past year. shes a great person but sometimes i just feel like we dont connect sexually. its not her fault though, we have busy lives and she has to work long hours every day. one night i was at this bar drinking with some friends, and there was this girl, elena, who sat next to me. shes in her mid 20s, very pretty, but also knew all about my engagement and was super flirty. we started talking and flirting and before long, she invited me back to her place for a drink. we had some wine and got even more into each other. elena leaned over and kissed me deeply, running her fingers through my hair. i kissed her back with everything i had, feeling my body respond instantly. our hands started exploring everywhere, and before long we were in the bedroom, fully naked. she climbed on top of me and we made love for hours that night. it was intense and raw and felt like a release after months of unfulfilled desire. her body against mine, fingers deep inside me, lips sucking at my neck... i couldnt help but feel guilty as the moments went by but it was too late. the next day was one of pure dread. i called rachel and tried to explain what happened but it felt hollow. how do you even apologize for cheating when its not just a slip up? im here now trying to make sense of this, why did i cheat on someone who treats me so well? can anyone help shed light on my thoughts or actions here?