well... i think is a common mistake to think as some poeple said "you have some 20000 days"... who knows? Death is stalking everybody. Graveyards have not only old people, you know. Every one of us can drop dead any moment. I have nothing to ensure me i will ever finish to write this post. Any thing we do can be our last act on this earth. Remember it, and act accordingly.
When youre in a bad day, think "this day may be my last one. would i like to waste it feeling bad?" or, in the words of Don Juan:

"Death is the only wise adviser that we have. Whenever you feel, as you always do, that everything is going wrong and you're about to be annihilated, turn to your death and ask if that is so. Your death will tell you that you're wrong. That nothing really matters outside its touch. Your death will tell you, 'I haven't touched you yet'." (Journey to Ixtlan, by Carlos Castaneda)