It's impossible to say how you'll react until it happens to you. I've been a hypochondriac my whole life and have also been so terrified of death. My husband asked me once why I was so afraid and obsessed with death and I realised its because I just love being alive! The smallest things give me so much pleasure- a glass of wine, listening to the birds in the garden, watching the first flowers of Spring appear, a hot bath at the end of a long day. I could go on forever! So for me I think it's that I'm really happy and can't bear the thought of leaving this world.
SO it came as a shock two months ago to be diagnosed with kidney cancer at the age of 39 with two small children to look after! Of course there were many moments when I was terrified but in general I was surprised how calmly I dealt with it. I was much calmer being diagnosed with cancer than I was 4 months ago when I was obsessing with MS the whole time. Then I couldn't sleep or eat and was a nervous wreck! Being diagnosed with cancer I had this feeling of calm acceptance a lot of the time (not all of the time believe me!! I got scared and cried every day too).
I spent hours thinking about death and at times I even came close to accepting it might happen soon. We are all mortal, the only thing certain in life is that we will die and having cancer made me accept this and feel at peace with it. The issue that caused me most distress was leaving two small children without a mother. I don't know... I was lucky in that I was always told there was a good chance I could be cured. Maybe I wouldn't have felt so calm about it all if the cancer had been advanced.
So I don't know how you'd react but you might be surprised at how strong and brave you can be when you have no other choice. I definitely realise how precious life is now and I want to deal with my general anxiety so I can make the most of however much time I have left (hopefully decades but you never know what's round the corner).