Among the many valuable lessons my parents taught me was "don't obsess over things that are out of your control." But that's easier said than done, and there are times when I'm frustrated by my own inevitable mortality. It would be less emotionally stressful if I believed in some sort of afterlife, but as has been discussed at length here many times, you can't force (genuine) belief.

Probably my biggest coping mechanism for this matter is taking some degree of pride in my accomplishments (for all that they probably carry no real weight beyond that satisfaction). If I can look back on my life and honestly believe it was well-lived, even though I surely could have done so much more, that will be enough.