Lately, I've been thinking about a metaphor that I think works on
multiple levels, and explains why all this thinking is worthwhile.
Fretting about mortality is like a forest fire; that sleepless, anxious, restlessness is like mental burning, and it can leave you in ashes.
There are different strategies of dealing with this situation. One is to constantly be on guard against any and all sparks, never letting trigger thoughts dwell in your mind. The trouble with this strategy is that you can never completely isolate yourself from reminders of mortality. Ned Flanders loses his wife on the Simpsons, a song lyric refers to time passing, or it hits you can guesstimate how many springtimes you have still to see-- there are far too many potential sparks out there.
A better strategy is to learn how to bring in regular rainstorms over your mental landscape, to make it less likely than any spark will start an out of control fire. You can get this 'rainfall' by learning comforting philosophies, by adjusting your attitudes so that you make peace with death-- once you've done that, these little sparks have no chance.