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Phrase(s): *one’s own worst enemy
Fig. consistently causing oneself to fail; more harmful to oneself than other people are. (*Typically: be ~; become ~.) • Ellen: My boss is my enemy. She never says anything good about me. Jane: Ellen, you’re your own worst enemy. If you did your job responsibly, your boss would be nicer.
McGrawhill's American Idioms And Phrasal Verbs