With increasing age, 60+, I have found the amount of backward sweep in my handlebars alters any shoulder/back pain. I now need 20° or more backward sweep to achieve comfort in the shoulders/back.

I also now ride with Ergon GP1 handlebar grips, the ones with a flat platform at the rear to spread the load on your hands.
I had my good bike stolen earlier in the year as I rode to a cycling rally. I had to use my old bike for the rally and surprised to find that I was in real hand pain using the old tubular rubber handlebar grips.

It may be worth trying some different shape or width of handlebars, plus some Ergon grips.