Seneca, Letters to Lucilius Letter 5

Though thou shouldst be going to live three thousand years, and as many times ten thousand years, still remember that no man loses any other life than this which he now lives.

Throw away all things, and retain these few only. Further, remember that every man lives only the present moment; the rest of life has been already lived, or is uncertain.

Do not disturb thyself by thinking of the whole of thy life. Let not thy thoughts at once embrace all the various troubles which thou mayest expect to befall thee.