Seneca: Begin at once to live, and count each day as a separate life.

Source

From the book Moral Letters to Lucilius (Epistulae Morales), Letter 101; translated by Richard M. Gummere, 1917-1925 (Loeb Classical Library)

Also quoted as

Also quoted as “Therefore, my dear Lucilius, begin at once to live, and count each separate day as a separate life.”