Block with your left leg and left rein. Put your left leg just slightly behind your girth. Don't pull back with your left rein, just keep contact. With your right rein, keep it just barely open so the horse can get relief from the pressure on his left. Keep your right leg on to keep his body straight.
Try a slight counter bend and apply your left leg. The goal is the have the pick up the left shoulder and move it to the right. If the left drift is happening at jumps practice the above exercise at at small jumps until you feel your horse straighten.