What i as an occupational therapist will suggest you is , usually we all do work mostly in sitting position and most often as per your query you are not able to maintain that position longer enough to complete your tasks. I will suggest you to start with back strengthening exercises such as bridging with 2-5 repetition with 5 seconds hold and gradually progress to 10 repetition with 10 seconds hold while your hands are laying by your side. That will develop some strength in your back muscles and for building up endurance i will suggest you to gradually remove back support i.e if you can sit for 15 min. Without support with no pain try sitting for longer duration to which you can tolerate some pain or till the pain is initiated and take rest as appropriate (1/3rd of the time you sat for) and progress gradually.

Please consider your physician for medications calcium and vit.d and please make-sure you sit in morning sun for a minimum of 10 min. Do wear a lumbo sacral support or LS BELT if that may help you in walking. And STAY HEALTHY