I have been facing mouth ulcer issues for a long time. I have visited multiple doctors before, and every doctor suggested I might have some B12 deficiency or other deficiencies. I have taken pills multiple times and done some blood tests, but the ulcers haven't healed completely. They occur every month or two, and sometimes even within two weeks. Could you please suggest how I can identify what this symptom is and why it usually occurs? Please note that the pain is severe once the ulcer appears.