I get an ulcer inside my mouth, on my tongue and in my throat, from the last 8 years. It occurs often with severe pain. The doctor did all my tests like blood, urine, stool etc., but they all were normal. The doctor couldn't cure it. What should I do?

Recurrent mouth ulce. Do they heal and they reoccur? If the do more likely they are recurrent mounth ulcers.
Ulcer. It is highly unlikely that a lesion that has lasted for 8 years is an ulcer. You should consult on oral surgeon.
Evaluation Needed. If you have had this for 8 years, then it could be normal for you but you should get a second opinion to find out the diagnosis for your discomfort as it's not likely that it is normal for you.