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

Episode 251: High Skilled Sitting/Abdominal Bracing

Episode 187: Death by Chair. How Much Do Actually Sit?

T Spine
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