A wayfarer carried a heavy sack about which he complained unceasingly. From none could he get help or comfort.

And as he slowly journeyed, groaning under his burden, the Angel of Optimism came to him and spoke kindly, saying: "Brother, what does thou carry?"

The man answered in a surly way, "My worries."

The angel smiled pityingly upon him and said, "Let us look into thy burden and examine thy worries." And so they looked in.

But lo! the sack was empty.

"Why surely," cried the man, "there were two great worries, too heavy for man to bear. But yes, I forgot, one was a worry of yesterday, and so it is gone."

"And the other?"

"That? why, that was a worry of tomorrow, and it has not yet come."

Then the angel smiled with infinite pity, saying: "Listen! He who bows himself down under the worries of yesterday and tomorrow wears himself out for naught. But he who carries only the worries of today has no need of a sack for his sorrows. If thou will cast this black thing aside, and give all thy strength and cheer and courage to the things of today, real misfortune never can burden thee."

Wondering, the man did as the angel commanded. And as he took up his journey and went lightly, swiftly on, his heart and his hands were free to relieve many a brother wayfarer of his burden and to pluck sweet fruits and flowers along the wayside. And when he came at last to the setting of the sun it was with smiles and a song.

--source?