I understand your concern.
I would like to clarify regarding one thing as we get old parts of the body will loose their function slowly but not totally.
Assuming that you are takingvyour medication regularly since 15 years I can say that tour present condition may or may not be because of diabetes. It Mau be because of the medications you are on since long. This is also one of the way that the kidneys loose their function.
Inorder to avoid raises creatinine levels you can take cinnamon powder, ginger as daily routine and you need to avoid taking high amount of salt and fried and Flatty foods.
I suggest you that it is time that you need to consult a nephrologist and get it evaluated
not all GFR reports need to be taken at face value. There can be age related decrease in GFR which can be physiologic or expected. All you need to do now is to follow up regularly with your diabetologist and take his/her advice. Please check for protein in your urine and if it is positive then it might be time to consult a nephrologist. Take care.