What should i do?

My best friend slept with my fiancé . She has for a long time been telling me that my fiancé is not faithful but I didn’t believe her because she had no evidence. She’s supposed to be my maid of honor at my functions. Recently I checked my fiancé’s phone, went through his texts and my best friend is one of the girls he had cheated on me with. I confronted her and she said she was going to tell me and also she did it to prove to me that I deserve better than a cheating husband. Advising me to leave him. I haven’t yet confronted my fiancé. She’s begging me to forgive her. What should I do?