Your doctor is right that gradually the effect of anti diabetic drugs diminish.
Most of the oral anti diabetic drugs act by stimulating the beta cells of pancreas
resulting in production of hormone insulin
which is responsible for controlling blood sugar levels.
The day a person is diagnosed with type 2 diabetes
,more than 50% beta cells in his pancreas are dead and gradually as the time passes rest of beta cells start getting dead and within 5 to 10 years all the beta cells become dead.This is the stage when oral diabetic drugs totally loose their effect.
I think you can still do with different combinations of oral anti diabetic drugs along with proper precaution of diet with regular aerobic exercises
for 45 minutes a day.
You can consult your doctor and get the prescription of medicines revised.
Once you feel that oral drugs are not affecting,immediately switch over to injection insulin.
It is very important to keep your blood sugar levels in complete control to avoid future micro as well as macro vascular complications.