Betrayal on My Shift

I'm engaged to my high school sweetheart, Mike. We're planning our future together, yet here I am with tears streaming down my face because I betrayed him last night at work. It was all so sudden and unexpected, a coworker named Alex who's ten years older than me... He just kept showing up at my desk after midnight shifts ended... He knew how lonely I felt... The tension between us, it grew until we kissed for the first time in his car before I went home... It was like an addiction... Once wasn't enough. A week later, another kiss turned into more... What started as a distraction became a real thing... I feel so lost and scared right now... Mike will be crushed if he finds out... How can I go back to him after this? Am I the only one who feels guilty about this...?