If you want to keep her, pretend nothing has happened and continue fucking her as usual. Alternative behaviours (such as whining or threatening) are not only foolish but useless: If she likes the guy she will continue seeing him whatever you do. If she doesn’t like him, she will let him go. Don’t mention that you see other girls it’s useless as well and even counterproductive.
However considering your apparent emotional state, I second Rocha’s advice.
However considering your apparent emotional state, I second Rocha’s advice.