See my other comment and comments of others. It is by no means certain that you have 15 years. I googled life expectancy for polycythemia and quickly found that people under 60 have longer prognosis. You are way under 60. I don’t know how much that counts but I’m sure it does. As I said before the average life expectancy today, for all the people who already died is 77. It isn’t very different to the life expectancy for everyone with or without your condition.

Look into the risk factors. See which you have, avoid those you can control. Find a specialist, get a second opinion. Get treatment. But try not to despair. I am a hopeful person, particularly with science and medicine, but I see genuine reasons for hope in your case. E.g. You might find there are a lot of smokers in the studies which came to those numbers of 20 years. Don’t smoke? You are likely in the above average set. Do smoke? Stop and get into the above average set. Overweight? Lose it. Use your diagnosis to live a better life. All those things I say I will do tomorrow? You’ll be doing them today. Every time you feel sad about the diagnosis, strengthen your resolve to live your best life. Every time you look at your family and think about your diagnosis, use it as a reason to do, or plan something memorable. I’m sure there are other things you can do. Be proactive, take as much control as you can, but still do all of those things lie video journals and packing on memories for your family. Even if you live to 94 they will still appreciate it all and so will you.

Don’t give in to despair, don’t give in to the prognosis. Live your best life.