Hi,
Mouth sores reasons are several . It might be gastric issues, or trauma induced ,
vitamin deficiency, bacterial causes,hormonal. The exact cause cannot be pointed out. You can Use multivitamins for 10 days every month( it should have vitamin B complex and zinc essentially) . Also take probiotics like yakult it may be helpful for maintaining your stomach health. Eat less hot and spicy foods and increase the amount of water in your diet. These lifestyle changes need to be carried out. Please keep the mouth clean and (refrain from smoking, carbonated drinks , pan / guthka chewing if applicable)
Next Steps
for local application on ulcers you can use quadragel which is very effective use 4 to 5 times atleast for better results. mouth rinsing with chlorhexidine mouth was (for a week only)can be done additionally.
hope you get rid of the sores problem soon.
Health Tips
please get it examined by a dental surgeon for ruling out any other issues.