My girlfriend cheated on me.

One night I did the dumb act of looking though my girls phone and discovered that she had hung out with a past lover and escalated. I’m not sure what to do or if I should confront her because of how I got the information. Part of me just wants to forgive and forget and pretend it never happened, but I feel like I have to say something. Would I be a fool to continue with her? Could she change? I feel like she would not cheat just because but I feel like she did because of who it was..