I "invented" an exercise that quickly stops my middle of the back pain.
I use an exercise rubber band. I stretch the band around the stair post,
grab both ends in my hands, and stand with the band lightly stretched
with the band on my left side behind me and my shoulders twisted at the
waist to an easy limit to the left. I then slowly rotate my shoulders to
the right, pulling the band to wrap around my waist, without moving my
hips, as far as I can rotate, then slowly rotate back. Repeat that a
bunch of times, then do the same exercise on the other side.
If I do this as soon as I sense the middle back pain, it quickly goes
away. My idea was to exercise the muscles that stabilize the back in
rotational movements. The motion sort of duplicates the motion I used to
use when paddling my white water kayaks. I did not have back aches when
I was doing that activity.
I started doing this after several weeks of PT exercises did me no real
good. You could ask your PT whether this might be useful for you.