If fistula, they are usually treated conservatively with time and nutrition.
Nutrition is the key element here.
Even though you are obese, you need extra calories, protein, etc. to heal the tissues. Perhaps a consult by a dietitian would be helpful. They will calculate your needs for healing accounting for your weight and any other problems. Most fistulas are treated with a small ostomy pouch if the drainage is high or dressings. Nutrition is also important here as well.