Mouth ulcers probably interest me more than they interest the average dentist. As a child I got these "canker sores"  on a regular basis, some reaching the diameter of a dime.  In my training and practice, I have searched pathology textbooks and researched literature at regular intervals looking for explanations and, above all, relief.  Most importantly,  for over thirty years I have observed the behavior of these painful  little lesions in myself and in my patients.
There are many treatments sold for apthous ulcers, most being ointments to cover and numb them. Many divergent ideas about their cause and treatment can be found on the Internet.   SO, here, on this site, I am offering the same guidance I would offer a patient in my office.  I'll tell you what has worked for me and many patients,

May your mouth ulcer disappear soon! 
                                                                 --Dr. Lynne Heckert