There's a lot of good advice in this thread. Accepting there is no true relationship is the first and hardest step but it's necessary to make the following steps easier. Your mother only loves you when she can see you as an extension of herself because she's only capable of loving herself - and even then that love is self serving and a projection of positive traits she has applied to herself. She feigns introspection out of manipulation. It's something my mom did a lot too, crying that knows she didn't always do her best and wants to reconcile then screaming what a brat I am seconds later when I don't accept. She has never apologized for anything. She won't. I will never even fantasize about it because it's so absurd to consider. There will never come a time when your mother considers how she treated you as wrong.
Unfortunately, you're stuck with her for the time being. I was in the same position. Your dad is out of the picture so there's no potential enabler but you and you've decided to no longer take it.
Grey rocking does work wonders, it's also difficult at first. It's important to remember her behavior is not a result of you or anything you've done but that you just happen to be there. She'd treat anyone else in your position the same, it's not personal no matter how much she makes it feel it is. Your boundaries are not negotiable just because she's your mother. As far as you're concerned now, she's just a mean stranger.
Grey rocking and eventual no contact will lead to some panicking on her end. She's losing a support, as she's lost countless others due to her behavior, but you're supposed to "owe" her for your existence. You're not supposed to leave. You've outgrown a cage you were born into that in her pea-sized brain was supposed to hold you forever. She's going to beg for a response - act out, lovebomb, threaten, and guilt. Do not respond to any of it. Responding means you care and you caring about her is all she has left.
Something else that's common is her seeking out other people to enable her behavior because she'll die without attention. My mom is currently targeting my younger brother as I'm no longer available. He hates her but is under her thumb due to extended family supporting her. I had to cut them off too. People who ask you to reconcile or "just talk" with her are not your allies. I've also had family do recon for her, telling her why I hated her so she could get rid of evidence and change the story. You do not owe anyone an explanation and I suggest you don't give them one if they're associated with her. People who see it for themselves will leave with you.
Once ready for no contact, secure legal, financial, and medical documents and high value or sentimental belongings. She will likely try to take them once she catches on. Know your rights, know where you went, when, and why. False missing reports, wellness checks, or involuntary psychiatric holds aren't unheard of. She will do everything she can to keep you there. Your goal is to not give her a legal reason.