Hi.. Welcome to healthcare magic. As you mentioned you have problem of mouth ulcer that may be because of your smoking habit or stress or trauma or other reasons. Diagnosis can occurs after clinical examination of mouth ulcer. This one may be a premalignant lesion . Consult a oral medicine specialist for clinical examination of mouth ulcer. Oral swab culture helpful for diagnosis. Try to quit smoking habit. Mention how long you have this ulcer in your mouth. Maintain proper hygiene of oral cavity. Avoid trauma by using soft brush. Topical anaesthetic gel helpful. Steroid topical can use at last if not resolve... Consult a dentist or oral medicine specialist for further diagnosis. Start a treatment as per reason behind it.. I hope this will help you. Get well soon.. Thank you.