You can't 'hold a mirror up' as you put it because these people will never change, and never see themselves as the narcissists they really are. This is because to do so would demolish the sense of self they have built up and unravel them as a person - on some deep, subconscious level they know that it's all built on sand and inside they feel empty and like a nothing, but they can't ever stop and drop the facade because all hell would break loose psychologically.
This is why there is there no point in trying to get them to 'see', and also there is a huge likelihood that if you do attempt it, they will turn on you (because you are fundamentally threatening their sense of self) and rage at you terrifyingly, demean you, and verbally tear you apart. It's a defence mechanism, and totally subconscious so they aren't even aware of it. But there is just no point at all, so save your energy and peace of mind.
My mum is like this, I have been no contact with her since Easter. I won't say it's been blissful because, well, she's my mum, you only get one and I'm devastated its come to this. I went low contact for years but it just wasn't working and she was still really messing with my head and being awful a good deal of the time when I saw her.
I have had to suffer guilt, the thing where they try and get other people to persuade you round (google NPD flying monkeys), her trying to get in touch, fear of other people finding out what I have done and hating me/thinking I am evil, constant second guessing myself especially when I just want my mum and then remember what she is like. It's been hard.
I also have gained confidence I could never have imagined, started going out and making new friends and just become almost unrecognisable in terms of how much I like myself, how outgoing and happy I am, how AT PEACE I feel all the time, calm and safe in my own skin. I didn't know how much she was bringing me down. I didn't see her today although I did sort a small gift and for DS (9) to drop it in with his Dad while I waited in the car. I felt comfortable with the choice, and I was firm and resolute which it turns out others respect more than if you are dithering, but it's taken a lot of months to get to that firm place of "No, this person is really bad for me and I'm not going there".
Sorry, I've written an essay but I just wanted to give my perspective on going NC with a narc mum. It is hard, I won't pretend it's easy but for me personally it has been intensely worthwhile in ways I couldn't have dreamed and I wish I'd done it 10 years ago. That doesn't take away the fact that it is a sad choice to make, one that no-one should have to make and I'm still dealing with the pain of it being necessary.
Good luck OP. 