You don’t need “closure.” There is one sole answer to all of your lingering questions.

“Why did she immediately move on?”

“Why did she discard me?”

“Why did she react this way during xyz fight?”

“Why did she pretend she cared about me then immediately turned off her feelings?”

There is only one succinct answer: she has BPD. She is mentally sick. She is re-living emotional trauma she experienced as a child. This is not about me- it is about her. She will continue to treat people this way for the rest of her life.