I'm so sorry, OP. I just lost my mum to cancer at 69. She was fit, healthy, slim, all the stuff she should be, just fucking cancer doesn't care.
Some thoughts, having just been through it and still dealing with things:
Tell her everything you want to tell her now. We didn't realise my mum was so unwell and there's stuff I didn't get to tell her. Do it now.
If she's able, have a gentle discussion about wishes for funeral, find out where her will is, stuff like her birth certificate. My mum left everything for us in a folder and it's made a horrendous time easier practically.
When the end comes, it might come quickly or it might not. I almost delayed going to see my mum because we were told it was days and weeks and it turned out to be hours, so if you're in a situation where you aren't sure if you should be there or not, be there.
And when the end comes, take the time to be present in the moment and give yourself the opportunity to take the time to sit with your mum after. There's no rush to get stuff done or let medical staff know or anything like that. Just sit with her, open a window, hold her hand, let yourself be present in that moment. The time for everything else comes after, but there is something very profound about being with someone when they start their journey to the next phase, whatever you believe in.
My mum died far too fucking soon, but her death was wonderful. It was totally peaceful, she had excellent quality of life till the end, no drawn out weeks and months in hospital or in a hospice. She walked into hospital at 9am, expecting to be out in a few hours, and had died by 3am the next morning, utterly peacefully and without distress. I wish it had been in 15+ years' time, but we were very lucky that when the end came, it came quickly and peacefully. I really hope it's the same for you, because that thought sustains me every day.
If you are with her when she passes, you will most likely get flashbacks about it. I get intrusive thoughts about it every day, but I also recognise the privilege of being able to be with her and hold her hand as she died, and in time I know the trauma part of it will lessen.
If you have young kids and your mum is well enough, get her to record herself reading a couple of kids' books so you have a record of her voice and something for them. I had a breakdown a couple of days after my mum died because I was terrified I would forget what her voice sounded like, then I remembered she had sent me quite a few videos on WhatsApp of her sending messages to my two girls that I was able to retrieve and save and back up everywhere.
I'm so sorry you're going through this. It's totally shit. My mum died a month ago today and I am still in a total fog of disbelief and shock about it. Be kind to yourself. I've been physically exhausted since it happened, I didn't really realise about the physical manifestations of both anticipatory grief and bereveament grief, but I hope you have a support network around you. Please take help when offered - I've had to force myself to do that as I'm naturally a bottler and an introvert, but I've forced myself to accept the offers of tea and a chat, company, etc. and it really has helped. xx