You are probably suffering from Recurrent aphthous stomatitis. The cause for this is multifactorial major one being stress
(stress ulcers) and autoimmune related. I have mentioned various other contributing factors in one of my answers. So I would like to focus on treatment options you have. Apart from keeping the good oral hygiene as top priority, hydrating yourself as much as possible, buy DOXYCYCLINE
capsules. Break the capsule and take out the powder. Make a paste by putting some drops of water and apply it directly over the ulcers for 4-5 times a day for 3-5 minutes each. Repeat the procedure for 7 days. If you get benefited, your are lucky as recurrence is unavoidable in many cases. Next best and permanent treatment is By LASER
ablation. Excellent outcome in chronic cases like yours. Its costly treatment, can not be performed by any dentist
but specialist in this field. You can update me personally if any improvement noticed.