Your diabetes makes it harder to heal wounds for several reasons:

Poor circulation
If you have had diabetes for a long while, you probably have fatty deposits in your arteries that slow down blood flow causing poor circulation. Poor circulation can limit the amount of oxygen and healing nutrients that reach a wound.

Nerve damage
Diabetes can lead to neuropathy, a form of nerve damage that causes numbness in the feet. If you don't feel the skin of your feet well, you won't notice things like your shoes rubbing against one area and causing a callus to form on the bottom of your foot. The callus puts pressure on the deeper layers of the skin. In many cases, this pressure causes a blood blister to form under the callus. The blood blister softens the callus from the inside. The center of the callus then deteriorates and becomes an open sore.

Immune system issues
To heal a wound, your body needs to clear away dead and damaged tissue and build new skin cells. Your immune system handles this job. But immune cells do not work efficiently when blood sugar levels are high. The reasons are complicated. The mixture of enzymes and hormones that are made by your immune system seems to be off from what is usual. Another reason immune cells don't function as well as usual might relate to the water content of the cells. High sugar levels in the blood can cause water to soak into some tissues and seep out of others in unusual ways. If they don't have their usual water balance, cells may not function perfectly to heal a wound.

Infection
It is common for a wound to develop an infection, particularly in a person with poorly controlled diabetes. An infected wound has to be treated before it can begin to heal.

To help a wound heal, keep pressure off the wound. Keep your blood sugar under control. If your wound is in an area that has swelling (edema), such as the legs or feet, get treatment to reduce your edema so that your circulation can be at its best. If you see signs of infection, tell your doctor right away.