So sorry that you are losing your mum to cancer.  I can only tell you about my experience of losing my mum to cancer which was some years ago now.  It was my first experience of cancer  happening to anyone I knew.  I was living 300 miles away from my parents and when I arrived home I took my lead from my dad who as can be imagined was extremely distressed.   Mum was in hospital where she remained until she died.   This was in the 1980s and it was common place for cancerand other terminally-ill  patients to spend their last days in hospital; in fairness the hospital staff were very good.  This was before cracks started to happen in the NHS.  I am certain my mum knew she was dying from quite early on but dad didn't want us to talk to her about it.  I regret this now as I feel mum would have been more than happy to talk about it.  She did talk to me about "seeing" her brothers who had died in World War 2 and even then I would so liked to take the discussion further.  I don't blame my dad at all; he thought he was doing the right thing.  Anyway - this is about you and not me - take your mum's lead about what she wants to discuss.  Be honest with her.  I know this is horribly difficult to you when you have your own fears and distress but don't shy away from any subjects she raises.  Apart from that, just do what you are doing, making sure she knows how loved, she is and make sure she is comfortable (I am sure you are already doing these things).  If she wants to discuss what will happen after she dies go with this painful discussion.

My very best wishes.