Learn As if You Were Going to Live Forever; Live As if You Were Going to Die Tomorrow.

From Life of St. Edmund of Canterbury from original sources by Wilfrid Wallace, 1893. Quoting Robert Bacon (d. 1248) who in turn was either describing, or possibly quoting, Edmund of Abingdon, a 13th century Archbishop of Canterbury.

Learn as if you were going to live forever; live as if you were going to die tomorrow.

- translation of the original Latin

Discere scilicet, quasi semper victurus;
vivere quasi cras moriturus.