I do back extensions for pain such as you describe.
They have a machine at they gym which I call "My wonder back machine." If my back is hurting before, it is fixed when I am done with my exercises.
You sit in the machine with your feet resting on an adjustable length foot rest, there is an adjutable height roller which rolls against your upper back, and a pulley to variable weights.
I lean back against the roller, fully extending myself to a flat position. I start out with low weights and build up to 300 pounds, which is the max.
Of course, you would want to check this out with your PT, but it does it for me.
You can also do back extensions lying supine on the floor, raising shoulders and legs at same time.
We all start developing osteoarthritis when we are about 22 yo. Some faster than others!