My last ex. Found out too late that she has Borderline Personality Disorder.
Towards the end of the relationship, I would randomly wake up at nights with her standing over me, waiting for me to wake up so she could scream at me.
I hope she got the help she needs, but that was awful.
I wish I knew! I tried to be as supportive as possible, but they would go from “I love you more than anything” to “I hate you” so quickly.