(As) stubborn as a mule / (As) obstinate as a mule

Extremely reluctant or unwilling to change a particular opinion, behavior, or course of action, especially when faced with opposition.

Examples:

  • My toddler is as stubborn as a mule, and he'll throw a tantrum any time he doesn't get what he wants.
  • You'll never get Rich to change his opinion - he's obstinate as a mule.
  • I tried to convince Jake to go to the doctor, but he's as stubborn as a mule.
  • For four years, Henry pestered his parents to let him learn the trumpet. They tried to talk him into some other, quieter instrument, but he was stubborn as a mule, and now he has a trumpet.