Hello, fiveminutes. Yes. I can totally relate. My older sister, who was emotionally kind-of a mum to me, as my real mum doesn't do emotion or affection, died suddenly of a brain tumour on 19 December when I was 14 (this was a long time ago). I was simply devastated. It took me years and years to come to terms with it, and that bloody song 'so here it is, Merry Christmas, everybody's having fun' has always made me want to scream 'no they're not!'
I finally enjoy Christmas quite a bit nowadays as my daughters (11 and 8) get excited (though the older one obvs knows about Father Christmas!) but it has taken years and years and years to get to this point, and I still get overwhelmed with emotion sometimes. I totally get what you say about people expecting you to be happy. It's so hard.
My advice to you would be to cry if you need to. I didn't cry for years, bottled it up, soldiered on, and then fell apart a few years ago. I got through it eventually, but repressing feelings isn't ideal! Yes, your mum wants you to enjoy Christmas, but your feelings are important tol, and it"s normal to feel sad when someone you love deeply is seriously ill. Let it out if you can, but also try to enjoy the moments you have with her. It sounds like you have a very close relationship, which is lovely.
Sending you lots of un-mumsnetty love and hugs xx