is 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...