I have big breasts and pain in the upper and middle back. What do you suggest I do?

Large Breast with. back pain. If you are already not using a custom fit bra with under wire support then do that. Some times a sports bra may also help. I see you have had a gastric sleeve and hopefully that is helping with your overall weight. Ultimately, you may need a breast reduction. Consult a Board certified Plastic Surgeon for an evaluation. In your case insurance may pay for this,