Find the position of the handle which is most comfortable for you. Along with that you need to start working on the muscles around the shoulder blade. You probably lack static control and the small muscles which give you stability are getting fatigued and giving you pain.
Thorough assessment needs to be done. Working on weight loss will definitely help
Next Steps
See a sports physiotherapist (if in Mumbai find Flex Physiotherapy Centre, Babulnath).
Health Tips
Avoid long rides till you see the concerned person. it will cause unnecessary irritation of the tendons.