Ok. Your video totally helps. The issues is with your left hand placement. I have even smaller hands then you do, and you have to learn to roll your wrist in towards your body so you keep leverage. Its like gripping a baseball bat, your technique looks like you are holding a bat by your fingertips and thump. Not much power. Start by putting the neck in the crotch of your thumb and first finger. And then curl your fingers over. This will give you more torque, more power, and less tension on the thumb. OH, and you might want a few private lessons from someone good.
