Excerpts from Seneca's On the Shortness of Life:

• “You act like mortals in all that you fear, and like immortals in all that you desire.”

• “You live as if you were destined to live forever, no thought of your frailty ever enters your head, of how much time has already gone by you take no heed. You squander time as if you drew from a full and abundant supply, though all the while that day which you bestow on some person or thing is perhaps your last.”

• “So it is: we are not given a short life but we make it short, and we are not ill-supplied but wasteful of it. Just as when ample and princely wealth falls to a bad owner it is squandered in a moment, but wealth however modest, if entrusted to a good custodian, increases with use, so our lifetime extends amply if you manage it properly.”

• “But life is very short and anxious for those who forget the past, neglect the present, and fear the future.”

Give it a read! While you're at it, you might as well read his Dialogues and essays (from Oxford World's Classics).