But maybe not as stressful as allowing her back in. Steel yourself that this time it's for good, whatever she throws at you.
Of course she hates it if you go NC. She's a narcissist, it's part of the definition that the (her) world and everyone in it must revolve around her. Rejection is one of the things a narcissist fears most because it means someone isn't playing to the rules the narcissist sets for them, they have lost control, and they will do anything to take it back. Well when I say anything, becoming a nicer person obviously isn't included. They will use guilt, blackmail and threats. In a nutshell, they almost lose their minds at the outrage of NC.
Grit your teeth through the worst parts and it will get easier in time.
The Narcissist in Your Life: Recognizing the Patterns and Learning to Break Free