Why do you regret saying no to the person that asked you out?

Not the person who asked me out, but a girlfriend who cheated on me. She kissed my best friend, and obviously I was so mad I left her. For weeks after she begged me to take her back, that she was sorry. I ignored her and was so angry, but now that I’m older I realize that she was genuinely sorry and wanted to work through things. She’s married now, and I’ve been single for years. Big regret there