Oh god, you poor thing. Hope everything goes ok for you.
When I was diagnosed with cancer, I told my kids the truth from the beginning. I had to, because I was so down, I couldn't possibly have hidden it. The news didn't seem to affect my 13 year old too badly. Perhaps she internalised it. My 11 year old was very upset.
I said that I was so upset because I really didn't want chemo as that fought the good cells as well as the bad, and that I didn't want to feel poorly because of that. It shifted the focus from the prognosis. I also said that hopefully the treatment would make me better. I said that cancer is a very scary word because, years ago everybody did die from it, but now many people are cured, and hopefully I would be one of those. It was likely that i would be. I also said that one in three people get it at some point in their lives and most of those are cured.
So whilst I didn't say I would be 100% ok, I focused on the positive and deflected my concern onto the chemo iyswim. I have now got the all clear.
I hope that everything goes ok for you. I know exactly how you feel. My concern was for my children, rather than me. It is scary, I know.
If your prognosis is bad, then take advice from the Macmillan nurses or a support line for your type of cancer.
Wishing you all the luck in the world. 