I understand how difficult this is as people have a cult like response to any parent claiming they're the victim of their unreasonable child. It seems like they cannot understand behaving in ways that force you to behave as a parent towards them is selfish and immature.

I'm thinking of all the times I had to tell my nmom like one speaks towards a child, "No you're standing to close to me." "What do we do when we don't know something, we ask questions!" While they acted completely mystifed by basic social behaviors a preschooler understands. Like, not to make big eyes and look around like a lost child in a mall about to break down, because they were told not to stand right next to me in the kitchen.

And you get told you're mean why? It's mean to constantly humiliate your daughter when you have shown you can behave like an adult in other situations. To put them in the position of having to be "mean" because you're literally dancing in the middle of the mall like a happy clapping toddler in an adult's body.

My point is you can try your best at this, but if it's mental illness to behave like a monster, or to act like a forever child, that comes across more as enabling by declaring it a mental illness they're a victim of.

They can change their behavior, they just don't want to. No differently than a stubborn child making sad mommy abused me by saying no more chocolate eyes and sulking around the mall, or a child throwing things off a shelf until they get what they want. That may technically be a form of mental illness, but it's also behavior they refuse to stop engaging in. It's not someone with a brain injury, op's mom likely could've not said such horrible things. However like a tantruming child shreiking I hate you mommy they lashed out.

If you ask me the only response to that behavior is no response, because they want a response. Some people may call establishing a boundary the silent treatment. Those people haven't had to deal with someone who the moment they see attention being payed towards them, they react like a 1 year old. The moment they're not being coddled they act up. The only way to counter that is extinguishing the behavior by not reacting to any of it. "That's nice!" "Wonderful!" are praise statements. They need to learn you will be heard when you act your age.