to me it sounds like she has a huge ego working overtime. as long as you gave her your full attention, she'd "grace" you with hers - until that wasn't enough. nothing was ever enough to fill the all consuming greed of the ego.
you got sucked into the times when she would mirror back attention - when she'd be "charming" and lively. she'd say whatever seemed to keep you hooked and continually feeding her ego.
but here's the thing - compassion isn't a sometimes thing. people who are truly compassionate and empathetic towards others are that way consistently, not when it suits them. she could only pretend to be those things.
i am glad to hear you finally hit your bottom on her unacceptable behaviors. i know it hurts.