What is the best way to heal as the scapegoat from a mother with narcissistic personality disorder?

Therapy: If you haven't been to therapy, you should. Being scapegoated in a family alters ones perceptions of otherwise healthy relationships. Learning new ways to manage the relationship with a narcissistic family member also helps protect you and develop better personal and emotional boundaries.

See a professional: If healing needs to occur in your life a professional could guide you on this. Obviously, you'd like to respond differently than you do now in your interactions with others. Healing takes lots of practice over long periods of time. Get professional guidance to ensure that you're practicing is on the right track.

Narcissism: The cornerstone of therapy in this context, which, in my opinion should be psychoanalytic, is to realize that mother's narcissism has nothing to do with you, i.e., she was well on her way on this bath before you were born. Dealing with the aftermath and accompanying scars is a long-term and often herculean undertaking, but well worth the resulting emotional freedom.