I think you might want to talk to your veterinarian about using a prescription diabetic food. I would not choose Pedigree in your situation. What kind of dry food are you using?
Along with giving insulin I would suggest you try Dia-IonX because it helps to control the symptoms of diabetes and it's therapeutic mineral base helps activate the enzyme systems in the body that help with healing. After giving Dia-IonX for a while some owners have reported that they have been able to reduce the insulin dosage and some have been able to stop giving it because their dog no longer needed it. However, each patient is unique and has its own set of circumstances. One cannot stop giving insulin until the body no longer requires it. In addition give Canine Probiotics with each meal, MSM, and Omega 3,6,9 daily. Dia-IonX will help the body better utilize the other supplements for a faster response. In addition make sure that you offer sufficient water free of chlorine and fluoride.
A regular exercise schedule is also helpful in moving glucose from the blood and into the muscles which will reduce the blood glucose.
It is important that you monitor the blood glucose levels so that you can ask your veterinarian about adjusting the insulin dosage when necessary.
Please let us know how you progress and if you have any other questions.