You are right Netball, it does hurt, period.
Can I suggest that you call Nana? If you can't remember the Xmas conversation, call her, and make a new conversation you can remember. Will your Parents allow you to make the tollcall?
Is Nana on the net? If so, you can email her, and store her replys on your USB stick, it would be really cool to collect her replies and in later years re-read them.
I understand the intense emotions you are going through, beleive me when I say, it is completely normal to experience them.
I love the fact that you are wearing the greenstone she gifted you with, (I know the significance of that, I live in Wellington too). Keeping that near your heart, will always make you connected and close no matter what the outcome of her treatment is.
I beleive Nana will be watching you grow up, is that something you have thought about? If it is a belief you have, it can bring you alot of comfort over the years, I also truely beleive, that when you have loved one so deeply, you reserve a place for them in your heart that is exclusivly there special place.