You did not say if the problem was diabetic retinopathy or diabetic cataracts.
As far as getting the blood Sugar down chromium is the most important. Chromium polynicotinate is the most effective. Chromium picolinate is better known but 300 times weaker. Recommended dose for chromium polynicotinate is 200mcg 3 to 4 times daily.
Magnesium is also beneficial for both production of insulin and to prevent insulin damage. Magnesium malate is my first choice followed by magnesium citrate. Avoid magnesium carbonate and oxide. Recommended dose is 300mg 2 to 3 times daily. Do not take it with calcium for this purpose.
I also recommend the herb gymnema sylvestre, which is both a Sugar blocker and it increases insulin receptivity by opening receptors. It should be taken about 15 minutes before meals.