Hi, we had a diabetic dog, Benji who was partially blind and we managed to keep him going with diet and insulin injections for over 3 years, we always said when he no longer had any quality of life we would know what to do.
He became ill and went downhill very fast he was not walking properly and seemed upset that we had to carry him to the garden to do his jobs, his appetite went and it was obvious that the joy in his life was gone.
He had a beautiful life with us running on the beach, surfing with the children and travelling to Ireland for holidays, we made the decision on a Sunday morning in July when the sun was shining and he was finding it difficult to go to the toilet, it was heartbreaking but the vet said it was the right time and he went to sleep in our garden surrounded by love in our arms, a fitting end for our dear friend.
It is so hard, but it sounds like your dear friend is now suffering as well, I think you will know what is the right thing to do, as it is too upsetting to see them suffer and they do not deserve to, and although we now have another dog (rescued from Greece!) Benji will always be in our hearts, as your little dog will be in yours.
Best wishes and Kind regards