Installing a mirror on the wall near where I play was one of my best cello moves in a long time. However, it now seems that all this time I thought I was drawing the bow horizontally, really I wasn’t. It looked more like a 30 degree angle!
As a result of this distressing revelation, I now am watching myself in the mirror, trying to train my RH to draw the bow horizontally across the D and G strings. In my frame of reference, it feels like I am moving my hand in an arclike pattern, but my bow at least is moving horizontally. I wish I had done this earlier.