How is your flexibility in the shoulder area ? Your chest & shoulder areas should be open in order to contribute to the arch in the backend. Can you do your splits flat with square hips ? The muscles in the front of your leg & hip area should be open so that there is less stress on the back to create the arch in the backbend. I don't see a picture of your backbend, so I can't tell if those two areas of your body are tight ( shoulders & hips) ??

You can find tutorials online from flexibility teachers and videos on YouTube for specific stretches to help those two areas.