You want a diagnosis? I had one of those as a child/adult (I cut her dead when she attacked me with a weapon and I then found out that she had attacked my DD1).
A horrible, horrible person with
A personality disorder - Narcissistic Personality Disorder
A second personality disorder - Antisocial Personality Disorder
PMDD/Postnatal psychosis
Drug induced psychosis
Rage. Pure, unadulterated, rage.
There might be a tiny scrap of a negative childhood experience somewhere along the line, but don't be fooled into thinking that it could in any way be a reason not to stay the hell away from this creature.
Some people are just wrong. It's why we have fairy tales, myths and legends about monsters and demonic possession - it's how we rationalised people who behave so abominably, that there had to be a supernatural explanation for it, especially the way the eyes and face change; anybody who has ever been on the receiving end of this knows exactly what the face and eyes are that you describe, clear as day, even many decades later.
You can't and won't change into her. You may very well have cPTSD, reactive depression, anxiety - none of those make you remotely like her and are, quite frankly, hardly surprising after a lifetime of abuse.
In short, my diagnosis is that she is both mad and bad. You, however, aren't like that - you're traumatised.