Longbows, especially those with straight grips, can be sensitive to how you place your hand on the grip. I get my best results when the pressure stays on the base of my thumb. If you turn your hand, palm up, the crease that goes from just above the middle of your wrist to the middle of the webbing between your thumb and forefinger is called the lifeline. Try to keep the pressure of the grip on the thumb side of the lifeline.
I've heard some refer to this as the 'luggage' or briefcase grip. If you hold your grip like you would carry a briefcase or piece of luggage, you'll find it will rotate your thumb clockwise, when viewed from above, around the grip for a right handed shooter. Opposite for a lefty.
It will feel strange at first, but it's a way to insure the pressure line from the grip is on the base of the thumb. Pressure on this location puts the grip in alignment with the neutral position of the wrist and bones of the forearm reducing or eliminating torque on the grip.