You should video yourself and see if you are leaning a lot to the left. I imagine you're too focused on your left hand. Look up at the ceiling or out to the right more. More your body with the music and don't get "stuck" in one position. Definitely try a cushion support or something else to adjust your sitting position a little. Mostly, I would suggest "sitting as if you are standing." To do this, stand up and feel the natural position of balance that your body intuitively forms. Adjust your balance in every direction and feel that balance become disturbed. Find the center again. Then feel the weight of your upper body being transmitted into your legs to the floor. When you sit, imagine yourself still standing, but transmit the weight of your upper body not into your legs but through your sit bones into the chair. Imagine yourself still standing, with the same feeling of balance you had when standing.
Hope this helps.