Thoracic spine back pain following bridge training

I have a reasonably good elevated bridge per the stretch series. I can't do all of the exercises that follow (push-ups, hip taps, etc), but my shoulder extension/mobility is good and I have no pain or discomfort anywhere when training the series. Several months ago I started developing somewhat severe muscular back pain running parallel to my thoracic spine. Lower trap/posterior serratus territory, middle/lower thoracic spine. It got to the point where it was waking me up from sleep after 5-6 hours. Like, bad. It was only ever muscular. Getting up and moving around, someone massaging the area, and rolling it out alleviated the pain and my day would be normal, but it would take several days to go away completely because each morning after sleeping it hurt. I always sleep on my back. It was always worst in the morning upon awakening and basically gone 30-60 minutes after that.

PT and Sports Med MD said it was likely instability caused by the aggressive spinal extension, which was causing the stabilizer muscles around the area to work overtime and become sore and tight. I literally had to stop doing extension work altogether and I only recently started VERY slowly doing any of it, still with some residual discomfort. I've never had muscular pain anywhere else from stretching, only with back extension. The confusing part is I feel zero discomfort when I'm actually doing the bridge series. None. It's not easy, but I do the moves and when I'm done I feel fine. Then the next morning (and a few after that), I'm in what for me is a serious amount of pain.

My question is how do I scale back by bridge training to an acceptable level if I don't feel discomfort DURING the actual training? I don't FEEL like I'm pushing it too hard but I obviously am. So I don't know how to adapt my training.

There are  essentially three variables you can experiment with. 

Time, Depth & Intensity

To work with time, just reduce the time in the elements that work the problem area. Maybe start with reducing by half and every month adding a bit more time.

To work with depth you try to find a simple measure of how deep you go in the problem elements and then back that off a couple of notches. Perhaps that would mean feet higher up the wall in the bridge for example.

To work with intensity getting a little more support so your muscles don't have to carry as much of your body weight. So for the Thoracic Bridge, this could mean supporting more of you weight on a box by either bending your knees or perhaps resting the back of you knees on the box rather than your feet.

It's a process of trial and error but hopefully you can find some combination of the above that will ease you discomfort after training. 

Once you do find a good combination, you can slowly start to ramp up, by adding a bit more load to a variable month by month.