What should I do? I've been in a relationship with this girl for three years, and I'm uncertain about my next steps.

My girlfriend, who's 19 and has been with me for three years, has been working as a server and recently got involved with the cook. I'm not sure how to respond to this. She feels guilty about it, and she means everything to me. I did cheat once early in our relationship, and I'm left wondering if this is an act of revenge or something else. I'm feeling lost and uncertain about what to do next. She gave me a second chance, and I told her I would also give her one opportunity, but this really hurts. If you've been in a similar situation, how did it turn out for you? Did it continue?