Just ditch her. There's better girls out there.
Though, honestly, the very fact that you brought this up seems like you already knew that's what you needed to do, but just wanted to validate the choice. Don't require validation. Stand up, be a man and do what you think is right. Don't let yourself get suckered by a pretty face, because sometimes that pretty face can be two-faced.
You should be able to decide for yourself on this particular situation pretty easily. Seriously, you have the proof. The only thing being uncertain gets you in relationships is 'doormat' status.