He is his own worst enemy when he loses his temper.
be one's own worst enemy
idiomTo act contrary to one's own interests; to cause problems for oneself; to self-sabotage.
She keeps missing deadlines because she procrastinates, proving that she is often her own worst enemy.
By refusing to accept help from his colleagues, the manager became his own worst enemy and ultimately caused the project to fail.
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abound in one's own sense
To follow one’s own inclinations and opinions.
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ace up one's sleeve
A surprise advantage of which others are not aware.
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act one's age
To behave in a manner appropriate to one's age.
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act one's age, not one's shoe size
Synonym of act one's age (“to behave in a mature manner; to avoid being childish”).
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afraid of one's own shadow
Exceptionally timid.
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air one's dirty laundry in public
To reveal one's sordid secrets to the public.
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air one's dirty linen in public
Synonym of air one's dirty laundry in public.
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albatross around one's neck
A characteristic, possession, associate, or previous deed that hinders one's ability to succeed.