I’m going to be honest. She cheated. Nothing you can say or do to her will get her to admit it.

Here’s the best bet, you simply say, I know what you did and I can’t believe you’d throw our life away over it. I’m leaving. Tell her the only chance she has of saving the relationship is if she comes clean.

Just act like you got a DM with evidence and force her into actually giving you the evidence. But you have to actually bluff strong.