Are you happy now? Are you likely to remain so till this evening? or next week? or next month? or next year? Then why destroy present happiness by a distant misery, which may never come at all, or you may never live to see it? for ever substantial grief has twenty shadows, and most of them shadows of your own making.

Sydney Smith (1771-1845) English clergyman, essayist, wit

Memoir of the Reverend Sydney Smith, by His Daughter, Lady Holland, Vol. 1, ch. 11 (1855)
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Advice for fighting melancholy / depression / anxiety by "taking short views of life" and not borrowing trouble.