Hello,
Once a decay has started it has to be treated by a dentist, there is no self healing or home remedies to stop it. Yes you can reduce further decays from happening by reducing
sugar intake, following proper brushing routine. Please consult your dentist regarding the same.
Dental treatment procedures are not painful, a well trained professional can make the procedures less discomforting to the patient with proper counselling. Please get urself treated as an untreated decayed tooth can be more painful.