How to stop my teeth from decaying while having periodonitis.

Dental appointments : You need to see your dentist and have the periodontitis under control. Regular checkups and follow up appointments with the dentist are very important. Your dentist will look for decay and inform you.

Diet & Oral Hygiene: Two different things. Brushing/flossing help to manage gum disease mechanically. Tooth decay is primarily a chemical process. No amount of brushing will prevent tooth decay if your diet is high in sugar/sweets or acid (soft drinks/lemonade)--especially if you take meds that dry your mouth--or if you suffer from gerd, or heartburn. Fluoride rinses help to increase acid resistance of the teeth.