Firstly.. there are many causes for ulcers in the oral cavity.. visiting a dental specialist is mandatory for a good diagnosis. On the other hand, Vitamin C( found in citrus foods e.g. Lime, lemon etc) will be of help provided u have no gastric acidity etc Stress relief through physical exercise, sound sleep, yoga is always beneficial. Food habits avoiding junk food, oily and spicy, will help.. Mouth is a reflection of ur general health after all!