Personally, I think anyone in a Narcissist's life is there because they provide Supply of some sort otherwise you wouldn't be in their life. They need
Supply and if you're prepared to give it in a way that doesn't get you lost (even to yourself), then you can go ahead & give it.
My advice to you would be to inform all your family and friends that she has NPD and talk through the ramefications with them so they know what they might have to deal with from her behavior and the reasons she can't stop herself from doing them. They can support you while not getting hurt/defensive/shocked/retaliatory against her and cause her irreparable damage through Narcissistic Injury.
Work out if you are meant to be in her life or not. Try to work out if you are basing your own actions on what you would like to happen in an ideal world, you're a bit star-struck by her and are romanticizing the relationship or if you genuinely have 'agape' love for her - self-sacrificing, compassionate, forgiving, 'give your life for her' love. Don't lend emotionally what you can't afford to give as a permanent gift. You cannot make this about you as if you're acting on a savior complex, this is not going to be about you for a very, very long time. You may find it better at this point in your life to love her from afar but not be directly in the thick of it. You must decide.
If you do want to be in some sort of relationship with her, you will need to build yourself up first with all knowledge of what she potentially may do or say and how she operates and accept that she will do these so you don't have unrealistic expectations and the barbs will bounce off you. Don't ever try to match her tit-for-tat in her bad behavior patterns thinking you're cleverer than she is, that road leads to bombs exploding down the track because your conscience will wobble before hers will and she can turn around and scorpion you harder because she's been doing this all her life.
I am a born-again Christian so I have God's help but on a human level, I have made it through safely so far because I am really
useful Supply. That means I am respected for my talents, character and abilities and my consistent, non-wavering boundaries of morality and house rules for living and I am able to get through to my partner on a deeper level only because
of this combo - I am considered on an equal footing (unless he's in a post-Injury rage mood lol), I am desirable to him. Are you desirable to her?
My partner needs
to feel loved, accepted, safe from harm (from any source of harm, himself included), valuable in my eyes, respected for his intellect/body/insight/talents etc, held in a position of honor in my life (him first after God alone) and this is paramount - YOU HAVE TO ACTUALLY
LOVE, RESPECT, VALUE, HONOR, CARE & BELIEVE IN HER GOOD PARTS. Flattery keeps their false self topped up, which is necessary for a while but once you can move into a genuine state of being delighted to be with them come emotional rain or shine, you don't have to lie or exaggerate any more and that's when softening & growth can happen. He trusts me because I am worthy of his trust. Real love, humility & forgiveness over a sustained period of time - your life - is the way forward. You can make mistakes with her and get through it only if you have more good points on her tick list of you than bad. This is not a fairytale, this is a hard mission to undertake lol!
It's a big ask of yourself and no-one would hold it against you if you're not up for it.
I hope that helped, bit bossy, aren't I?