Worry — Interest paid on trouble before it falls due.

Harry "H. A." Thompson (1867-1936) American magazine editor, publisher

Article (1905-11-25), “Sense and Nonsense: Some Definitions,” Saturday Evening Post, Vol. 178
    (Source)

Often given as "Worry is the interest paid on trouble before it falls due."

Collected in Thompson's The Cynic's Dictionary (1906). (This should not to be confused with the column by the same name (and similar theme) by Ambrose Bierce, who had to change the column name and the name of his collected book to The Cynic's Word Book, and, later, The Devil's Dictionary.)

Variants (mix and match the parts):

• Worry is interest paid on trouble before it falls due.
• Worry is interest paid in advance on a debt you may never owe.
• Worrying is like paying a debt you don’t owe.
• Worrying about something is like paying interest on a debt you don’t even know if you owe.

This (or its variants), are often misattributed to Mark Twain; there is no record of it in his writings, and the earliest attribution found, is from 1936, a quarter century after Twain's death.

The phrase was used, but well after it was in circulation, by William Ralphe Inge.

For more discussion and history see Quote Origin: Worry Is Like Paying Interest On a Debt You Don’t Owe – Quote Investigator®.