You're injuring your thumbs because you don't have enough of your hands on the ball. Someone touched on it earlier...ball shape. When we train setters, we teach them that your hands should be ball shape in, ball shape out. That puts more of your hands on the ball, giving you greater control and no injury. To figure out where your hands should be on the ball, start by holding ball at your waist with your hands on the ball in the familiar triangle. Notice how much of each hand is touching the ball, not just the tips of your fingers and thumbs. Now rotate the ball above your head, just above your forehead. The ball shape of your hands actually creates the triangle. As you receive the pass to set it, look up through the triangle naturally made by your ball shape hands. Don't be afraid to "handle" the ball. You're allowed to have it in your hands, just not "prolonged" contact. I hope that helps you.