Seconding Flippinada -
Also. disabuse yourself of the notion that you will ever be able to have proper adult discussion with her about this where she will acknowledge what happened and you can move on from there. You can't do that with a Narc.
Yes. I think it's important to realise that there is NO conversation you can ever have with this woman which will make anything better. She's not a normal human being. All we're talking about is damage limitation, and that means being emotionally (or if possible actually) absent.
Also seconding SeaEagleFeather -
if you have children then you have a duty to put them over your parent. Children are vulnerable and destructive adults ... destroy. If that means putting yourself first over her needs, then that's what you need to do. Dealing regularly with a toxic influence places massive strain on you, which your children tend to pick up on - something that people who can't bear to go NC often try to deny. In a way it isn't about valuing you, it's about protecting your children because over stressed and ground-down adults are less good parents than normal-stressed parents.
You do owe it to your kids to do whatever it takes to protect them from toxic people, and you also owe it to them to protect yourself from toxic people. It's the oxygen mask situation: in an aeroplane emergency, they always tell you to put on your own oxygen mask BEFORE you put on your kids' oxygen masks. Because if you pass out, there's nobody to put the kids' masks on for them. You are their life support system and you owe it to them to keep yourself in good nick.
Other random thoughts which work with my mum (hoping some of them might work on yours):
Schedule your next contact/visit with her, and make it as far in the future as possible. My mum will normally stop badgering me for a visit if I tell her we're visiting on a specific date a couple of months away (and if she doesn't, I just say we can't make it before then). You can make the next phone call / visit progressively further and further away. She gets to tell people that her daughter's coming to visit, which is half of what she wants anyway (i.e. the appearance of a relationship, rather than the relationship itself).
Similarly with Christmas etc: it may pay to be proactive and start the subject yourself by saying exactly when and where and for how long you'll see her (and progressively reduce it for future occasions). You look as if you're the one making the running, but actually you're also dictating the limits of the contact.
I never ask her for any form of help, and never accept help of any kind if she offers.
I never leave my kids on their own with my mum.
It obviously helps to have some form of therapy (some organisations offer it at a cheaper rate if you have sessions with a trainee - a bit like haircuts! This is what I did - not ideal, but better than nothing. Also might be worth asking at Citizen's Advice Bureau or charities.)
The more people who know what your mum is like, the better. Keeping my fingers crossed you have a good support network of friends. You do need to be able to talk about how awful she is with someone.
Ageist comment alert: you're probably more tech-savvy than she is. I've adjusted my Facebook security settings so that I can choose what I make visible, and to whom. There are things I post which all my "friends" can view, and things I post which are invisible to my mum and to anyone who might mention them to her.
Similarly - I don't have an answerphone on my landline (and I'm considering getting rid of the landline altogether). If anyone really wants to get hold of me, they can leave a voicemail on my mobile - which my mum is reluctant to call because (like a lot of narcissists) she's incredibly tight. Also, with a mobile you might be able to block/unblock her number on a temporary basis whenever you feel like it, and if she asks you can tell her you're having problems with her network.
Basically, when it comes to controlling contact with her via any form of technology, you probably have an advantage.
Show no interest in anything she says. This makes you very unrewarding company for anyone, and in particular for a narcissist who lives to manipulate people. If she feels she's not pulling your strings, she just won't want so much contact with you anyway. My mum clearly doesn't really enjoy my company any more, which makes life much easier.
Good luck to all you children of narcissistic mothers out there (and remember: you're not children any more).