Seeing people in either/or terms, as either all-good or all-bad, partly defines narcissistic personality disorder and borderline personality disorder.
Establishing boundaries is the opposite of narcissism, because it requires the recognition that we are all a mix of good and bad qualities. (Or it requires the refusal to categorize as good and bad. Either works.)
If I look at myself in narcissistic terms, I will eventually starting acting like a narcissist. And similarly, if I look at others as either all-good or all-bad, I will eventually judge myself the same way.
Don’t ever believe this garbage about mental illnesses being incurable. It’s not true, and even if it is true, who cares? Any mental illness can be overcome. You don’t always need to be “cured.”
The first thought that pops into your head, your initial emotional reaction, that is not who you really are. Your first thought is just biology, or your mental illness, or your cultural conditioning. The second thought: that’s who you really are.
If you want to say no, that’s a good enough reason to say no. That goes for other peoples’ demands, but it is just as true for your own mindset, your own thoughts, your own emotional reactions.
I can say no to my initial emotional response. I can say no to my initial thoughts.
If you have that initial thought, that other people are either all-good or all-bad, or that you yourself are either all-good or all-bad, you can say no to that thought process.
You can establish boundaries with your own thoughts, just as you do with other people. You can demand consent from your own mindset, just as you do from other people.