Hi John,
So sorry to hear about the loss of your daughter and the pain your grand-daughter is going through. It's not quite the same for me. Sadly i left 23 behind me many years ago, but I only lost my mum a little over a year ago. Still your mum is your mum, whatever your age or her's when she passed away,so I do have some notion of what your poor granddaughter must be suffering.
I had suffered the loss of my dad many years before already, so had some idea what to expect when mum passed away, so perhaps that made it a little easier for me to deal with. I doubt your granddaughter would have lost anyone prior to her mum and, especially when they were both so young, it must be terribly hard for her to bear. I did feel it didn't matter anymore what became of me after mum died. I think I am a strong person too, but I don't think the person strong enough not to feel life is hardly worth living after the loss of such a close loved one has probably been born yet! It could be that she said what she said to you just to give you an idea of the depth of her pain, but did not mean what you fear. I would remind her that her mum loved her and would want her to move forward with her own life and enjoy it as best she can. She is only young and should try to live it well, for her mother's sake, as Karena says, to honour her memory if nothing else.
I too live in my mum's house with all her things around me still and I find that both comforting and painful at the same time, but, for myself, I would still rather be here than have to move somewhere else. I don't find it stifling at all, but I have heard others say that they found it helpful to move somewhere else to help them move on, so maybe that is something you could discuss with her to see how she feels about it. It is about her now after all, so she needs to do what is best for herself. I think it is best not to rush anything though and to wait a few years even before thinking what will be best to do in this regard.
It sounds to me, from what you say, that it is all still too raw for her to be able to make any decisions about anything yet, so she probably needs company and some kind friends who will listen to her and be there for her for now. Does she have anyone like that? Also, you might encourage her to write down her feelings daily. It helps get them out of your system abit to do that. She could also put together a photo album and memory box of her favourite pictures of her mum, her favourite bits of jewellery etc. Then she can turn to these, when feeling really low and the act of doing this could also be very cathartic.
Lastly, I would suggest visiting not only this site, but Let's Talk About Loss too. That one is specially aimed and run by people under 30 who have experienced loss at a young age. There she might be able to connect with people in a similar situation and age group as herself and that might make it easier for her to express her own feelings, than talking to someone face to face, if she doesn't feel ready yet to do that. They have quite a few meet ups too, so that might help her as well.
As Karena says, the main thing is to keep communicating with her. Are you able to visit or ask her to visit you? It might be nice to get together to do something to commemorate her mum.
Let us know how you get on. We all understand how she is probably feeling here.as we've all been or are going through it too. Wishing you both all the best and sending support.