Seneca, On the Shortness of Life
3.3 (tr. John W. Basore):
Look back in memory and consider when you ever had a fixed plan, how few days have passed as you had intended, when you were ever at your own disposal, when your face ever wore its natural expression, when your mind was ever unperturbed, what work you have achieved in so long a life, how many have robbed you of life when you were not aware of what you were losing, how much was taken up in useless sorrow, in foolish joy, in greedy desire, in the allurements of society, how little of yourself was left to you; you will perceive that you are dying before your season!
Repete memoria tecum quando certus consilii fueris, quotus quisque dies ut destinaveras recesserit, quando tibi usus tui fuerit, quando in statu suo vultus, quando animus intrepidus, quid tibi in tam longo aevo facti operis sit, quam multi vitam tuam diripuerint te non sentiente quid perderes, quantum vanus dolor, stulta laetitia, avida cupiditas, blanda conversatio abstulerit, quam exiguum tibi de tuo relictum sit: intelleges te immaturum mori.