To want to know my Mum's prognosis?

I feel your pain OP and I’m sorry for you both. My mum was diagnosed with bone lung kidney spine and spleen secondaries from breast cancer recurrences 15 years post treatment. I thought she’d be gone in 18 months but it’s 5 years later and we are on holiday now in south east Asia. She’s having immunotherapy as part of a medical trial and although she’s not cured and won’t ever be she is living and living well. Try not to think in timelines because often there isn’t one. Enjoy your time together and I hope there’s something out there for her too xx

I’ve always said in those circumstances I wouldn’t want to know if it was happening to me. If people around her know (you) then you could end up overwhelmed with emotion and blurt it out, or little looks you give her subconsciously will alert her to things.
You need to remember this is happening to your mum and she has a right to not know. It could send her over the edge knowing, and it would be even harder for her knowing that you did know. You would be showing signs of preparation too. Let her do this her way.

Lots of people lose family and friends completely unexpectantly with no warning, at least you know you need to make the most of your time with her now and you know that now is the time to make memories. Focus on that.

I would also really struggle with this, I'm sorry about your mum. You are absolutely not being unreasonable feeling this, we all deal with things in our own way and approach things differently. You are supporting your mum and recognise its her decision to make. So in my eyes you are doing everything right. Please don't feel guilty about feelings, it's actions that matter.

YABVU.
The patient has the right of self-determination and that should be respected.

I'm sorry OP. This is so tough. We went through something similar with my mum - she never wanted to acknowledge or say what is happening. I remember when I begged a nurse in hospital to please tell me if my mum is actually dying. It was her way of coping and while I didn't agree with it, I couldn't change it. I looked for signs and did speak to a family friend who was a palliative nurse, which helped. There are incredibly supportive boards on MN too. Saying all this, your mum might still have many years ahead, saying the cancer has spread doesn't always mean death is near.

I said yabu, but not sure you are in some ways! Obviously, the doctor cannot tell you even in vague terms unless your mum agrees, but I perfectly understand how frustrating and stressful being 'in limbo' is.
Make plenty of memories while you can, and keep a smile fixed to your face!
Take care

I don't think anyone is being unreasonable here, I totally understand why you feel the way you do and I would also struggle with the uncertainty, but it is your mum's decision and she has the right to not share her medical information with anyone if she chooses.