Assuming that you’re the same Hannah DeLadurantey that I just found on youtube trying out a Salzedo model at the Lyon and Healy factory, I think I can give some more advice. You seem to be moving your left elbow around alot, while not moving your right elbow. The fact that you are moving your left elbow but not your right indicates that this may be the culprit of your difficulties. Just moving my elbow around now, I can feel the muscles around my shoulder tensing up, and I’m sure the joint would get sore after lots of practicing. Try to keep your elbow movements to a minimum and see if that helps your shoulder. If it doesnt help, oh well! but it may the solution.
Just a note – you may find it hard to not move your elbow, because (as it seems to me from watching those two videos) you’re releasing the string by drawing your hand away from the harp. You might need to concentrate on articulating more with your fingers, rather than your elbow. Try releasing the string by bringing your finger into your palm, rather than bringing your hand away from the harp. What I’m seeing might just be the result of some strange angles or some optical illusion, but I thought it might be worth mentioning. If this is the problem, it may be more a more complex techinical issue than it is ostensibly.
I’m no expert, but these excessive movements would surely cause me some discomfort.
Best of luck, and good videos!