Good physio or good osteopathy will sort it out. Mine was intractable for about two years and then fully cured by a good osteopath in around ten sessions over a few months, with huge improvement from the very beginning.
Once the capsule is punctured, with a steroid injection let alone with surgery, the capsule may deteriorate further and never recover. The balloon of fluid is not meant to be punctured, cut, or have foreign substances injected in it.
And this has probably been mentioned before but as a diabetic normalising your blood sugar will help immensely to get your shoulder to heal itself.