..My Secret Engagement Night...

I met my fiancé at work and we started dating right away. It was great until one night before our engagement party, I cheated on him with his best friend who he set me up to meet... We were all supposed to be having fun, but the moment my hands touched [his name]'s chest and we kissed, it felt so right. It's been months now, and I still feel terrible about it.... Should I tell him? What should I do...