My Mother Acted Like A Child - What was wrong with her?

My mother, for as long as I'd known her, and according to my aunt - her whole life, has been mentally, emotionally, and socially below average. I never had a good relationship with her. She was completely selfish and self-centered as a child is. It was always "me, me, me." She never acted like a mother. (I'm trying my best to put this factually and not emotionally.) As a child, she was always jealous of me. She was in constant competition with me. Whenever my Dad would buy me new clothes, or dinner, or spend any kind of time with me, she would get jealous. I would always hear "Well, what about me?" After a certain age, I surpassed her emotional and intellectual level, and it was impossible for me to communicate with her. We would fight ALL the time. Throughout my whole life, she would tell me she hated me, that she wished I was dead, and would just scream at me and hit me. And this was mostly before the age of 10. After that point, I started yelling back.

She was also an extremely sensitive person. She cried a lot. Got offended a lot. Was extremely insecure. She was also possessive over my dad. (Not just from me) She was ENTIRELY dependent on my Dad. She always felt she was the victim. She would "play sick" whenever she didn't want to go somewhere. And even the way she talked was childish. She used words like "Milkie" when there were no longer children in the house. My dad unfortunately is blinded and just accepts that she was "different" and "needy" but she took it to a whole new level. She never really had a job and refused to get one. She had no hobbies or goals or friends or anything. And the worst component, she was absolutely the most embarrassing when it came to social situations. Think of a really old lady (which she wasn't) who is mad at the world and feels like it owes her something, and multiply that by 10. She didn't know how to talk to people. She yelled at strangers. Shared inappropriate information. And acted like whatever she wanted was right and demanded it all the time.

And now that I'm an adult, although I knew we couldn't communicate on the same level, I tried to mention how she treated me, and she acted like she had no idea. In fact, she denied it. This has greatly stunted my growth into adulthood and a few months ago she passed away(hence my inconsistent tenses) from cancer. When you're a child, you accept the world you live in as that's how it is. But it wasn't until recently I realized that something was wrong with her. All I could get from my aunt(who is probably the most sane person in my whole family) was the following:

1. She was born a blue baby
2. She stayed back in school 2 or 3 times
3. She never had any friends
4. Her parents coddled her because she was "special"

*She also had a car accident when I was 3 yrs old, which caused her some brain damage and caused her to be physically handicapped for pretty much the rest of her life. (Wheechair, to walker, to cane) Which all would have easily been my first explanation to the cause of it all had my aunt not confirmed she was always like this.

What was wrong with her?

Maybe some trauma. My mother too had some characters like of yours.
When she spoke or cried (only for herself, of course) like a children (?! brrr) and the moment after screamed like a big monster were really horrific.

It seem to me something of borderline, narcisistic, sadistic, depressive..

Im happy that usually this type of bitch dies for some phisical deseas and yours too, often tumor, but she already had it in a very young age, for the type of (stomach) tumor, but you know, never late for another.