I've been playing for 30+ years but hit a ceiling with my technical skills long ago. The answer was in the course: I am not holding my viola relaxed at all. My neck is rather long and my head way too much tilted. I experimented with different shoulder rests but none of them solved the problem. One of them even requires a tool to move the screws. My next step will be to go to the luthier and try a chinrest that is located over the string holder.