Hi thanks for the query.
The complaints of pain in the middle of the back for two years and its presence on and off suggest that mostly it could be mechanical in nature. As you had mentioned some thing to do with the posture it is true. I would advice you to consult local orthopaedic surgeon
who might rule out other causes of back pain
. If there are no other major causes of back pain i advice you to learn some back and abdominal strengthening exercises from your physiotherapist and continue doing them at your home. There is no necessity to visit physio daily for the exercise sessions. Finally get good ergonomic chair which provides a good support to your back.