At our last lesson, we reviewed angle of the bows, since I kept coming down too hard on the strings during the Schroeder study. The main focus was on the back. You need to think of your back contracting like a butterfly and the rib cage expanding. To practice this, I have new exercise to practice double stops using this method. WE also discussed keeping the bow moving along a straight-ish trajectory, using the angle of the arm to maintain. And to avoid an angle that was too sharp, or hurting shoulders, tilt the cello towards the right slightly.
Above all,remember shoulder goes BACK, not up. Having the mirror should definitely help.