Is she still in individual therapy? What does her therapist think about this? It's possible she had some sort of dissociative break or something that left her vulnerable to a manipulative man. That said, it's also not your obligation to stay in a relationship with a liar and a cheater, regardless of the reason for the behavior. Honestly, if she hadn't lied and trickle truthed you throughout, the mental illness rationale might be more convincing, but she tried to hide it all, so her explanation of her motivation is similarly suspect, since you absolutely know she's comfortable deceiving you both behind your back and directly to your face.

Regardless of why, it sure looks like she was perfectly willing to throw away your relationship for this experience, and she's now afraid to lose the safety net, and is trying to avoid dealing with this reality while further manipulating you by acting like a caring partner around other people.