My Fiancé (25) cheated on me during her first company retreat. [cheating] [group]

Me (26) and my Fiancé lets call her Bonnie have/had a great relationship I really thought I found my soul mate in her. We met in college and have been togeather for 4 years already. I asked for her hand about a year ago she was the happiest ever to say yes. We have a super awesome sex life no history of fighting between us or anything, shes my best friend not just my love. Shes 5’3 with B cup tits natural blond with freckles and green eyes, and round glasses.