Hi. This is a very serious problem and you need to see a wound care specialist.
The treatment will include relieving pressure, antibiotics, professional wound cleaning, and perhaps surgery. If the treatment cannot be successful there is risk of losing your foot. Controlling blood sugar is also critically important. Please do not try to treat this at home alone. Get professional help. Controlling diet and avoiding all sugar will also be helpful, but alone cannot be enough.
Please come back and let us know what is happening with you and how you are going.