The Ready State Virtual Mobility Coach is like having a virtual Kelly Starrett in your pocket.
Daily Mobility Exercises by Dr. Kelly Starrett › Forums › General › Becoming a supple leopard (Page 27) Spinal Mechanics › Re: Becoming a supple leopard (Page 27) Spinal Mechanics
You need to keep seeing the physiotherapist because they are addressing the symptom not the cause of the issue. You need to change your positioning to get at the cause of the bad positioning.
Once you set the position you need to take maintain the positioning when moving.
Torque and Trunk Stability Part 1: How to Stand
You need to be more aware of your pelvis positioning so you are not over extended.
Sounds like you need to take a look at your sitting positioning
Episode 351: The One Joint Rule
There are several episodes if you do a search.
Here are a couple to start with
Basic T-Spine Mobilization
T-Spine Smash – Double Lacrosse Ball
T-Spine Plate Smash
T-Spine Overhead Barbell Smash
T-Spine Internal Rotation Smash