Keep your wrist as straight and relaxed as possible, bring your thumb closer behind the neck (in the middle, not above) to better distribute the pressure, and adjust the height of the guitar so as not to break the angle of your arm.
edited
Keep your wrist as straight and relaxed as possible, bring your thumb closer behind the neck (in the middle, not above) to better distribute the pressure, and adjust the height of the guitar so as not to break the angle of your arm.