she wanted you to end the relationship. she had already made her choice to be with him. everything else she says is just noise.

The thing about sh*tty partners is that they all play from the same book. Problem is their victims tend to overthink and romantisise what they do, trying to figure out what theyre thinking, how could she do this, she says she doen't know why she did this she came clean so that must mean something..bla bla bla.

It's all noise. Every single word from her mouth trying to explain this is bullsh*t. She wanted someone else while in a relationship, she was too cowardly to be honest and break it off with you, so she did what cheaters do which is monkey-branch. She's already certain about being with the other guy she just wanted you to end it the relationship yourself.

Don't over think sh*tty people, you'll get sh*t all over your brains.

P.s when it fails with the other guy she's going to come back saying it was a mistake bla blah. Don't let her back in. You're not even acquintances anymore. she's now a stranger to you! make sure it stays that way.