I see someone has mentioned parts, introjects and parenting and maybe some info will help the original poster -

"splitting" with borderline personality disorder has nothing to do with trauma, how one was parented, introjects, parts and so forth.

I have dissociative issues and one of my siblings has borderline personality disorder.

splitting with borderline personality disorder is where a person feels so extremely emotional that they cant think straight. they perceive people as good vs evil thinking.

heres the difference between my sibling and I to help you understand

ever reach that point in anger where you just want to throw something, punch someone or whatever.

well a dissociative person (me) shuts that down, goes numb based on their past. and then reacts based on past memories, emotions, experiences.

but a borderline personality disordered person (my sibling) does the opposite.

they feel so much emotion that they perceive the people in their life as good vs bad, saviors vs pure evil.

some places and locations call perceiving people as good vs evil delusional thinking because human beings are not one dimensional (just good or just evil)

for some people medications help stabilize the misfiring of their brains electrical impulses giving them a more stable thinking and less reactive life,

others therapy helps

and still others combination therapy techniques work.

with my sibling she is on medication to help with the delusions and in therapy to learn how to manage daily life, and her emotions.

maybe you and your treatment provider can talk about why and when you see them as evil vs savior and maybe find that emotional trigger and then be able to find that middle ground. it takes time.

if you are on medications maybe talk about your meds with them too.