I swear if this turns into bigger than it already is, I will stop reading. Man, she is just a cheater. Fuck her, I cannot put enough energy into this if this is the forced fucking drama author trying to pull.
It is not even that I do not think people do not change, but she has not. She just now single and regrets not having a decent dude on the hook. She does not care for you, and will not suddenly do so now that she is suffering, she is back for herself.
And even when she does change, do not have a relationship with her, it will just be mentally taxing and shitty on your end and hers. Let her find someone else.