This way of holding a brush will probably feel strange at first, but it encourages you to use your whole arm to paint, not just your wrist and hand. Turn your hand palm up, rest the paint brush handle across your fingers, from where the little finger starts to the first knuckle in your forefinger, then wrap your fingers and thumb around the handle. Paint energetically, moving your arm and shoulder, not just your wrist.
Also consider: How far down the handle you are holding the brush. Don't strangle it! Experiment with moving your hand further down the handle, all the way to the end, making a note of the results. Buy a long-handled brush and work at arm's length from your canvas.