be one's own worst enemy

Verb

be one's own worst enemy (irregular; see the conjugation of be)

1. (idiomatic, intransitive, irregular) To act contrary to one's own interests; to cause problems for oneself; to self-sabotage.
   • [Mr. Haven] objects to our making the Liberator a free, impartial, independent journal, allowing all sides to be fairly heard (as we invariably do) in its columns. He insists on making it (what it has never been) the official organ of the Anti-Slavery Society; […] The American Anti-Slavery Society...1861 March 29, “Reply of Rev. Mr. Haven”, in W[illia]m Lloyd Garrison, editor, The Liberator, volume XXXI, number 13 (number 1580 overall), Boston, Mass.: J[ames] B[rown] Yerrinton & Son, printers...
   • "We won't talk of that now," answered Mrs. Forbes. "Mark is his own worst enemy. Good night." True. No enemy however powerful and malicious, can so work a man's ruin as his own unbridled passions and depraved appetites.1870, Mary Dwinell Chellis, chapter I, in Mark Dunning’s Enemy (Standard Series of Temperance Tales; III), Boston, Mass.: Henry A. Young & Co. […], →OCLC, page 27