Your Own Mortality
My first time I seriously thought about it was when I nearly died from Guillain-Barre' in my mid-20s. I was paralyzed and my heart and lungs were too weak to work on their own...definitely thought I was going to die.
More recently after all my medical issues, losing my business, and my marriage I came very close to committing suicide. Now I try to primarily focus on the things I can change so while I am aware of my mortality, I really don't think about it as there is nothing I can do to put it off other than not be too stupid and being reasonably healthy