I am suffering from mouth ulcer since long.

Mainly it is due to Dyspepsia, consumption of heavy Antibiotics which kills normal Bacterial flora which makes vit B complex for our body.
It is also due to mental tension. I have seen so many exam going students having ulcers in the mouth and when the exam is over it is cured.
Mouth sores are common and rarely cause complications. You may need Coronoplasty (smoothen teeth edges) along with Multivitamin cap, for five days in the morning after meals & antiseptic ointment or gel on the area of ulcer until then.

Usually, the mouth ulcer is due to the low immune system. So take supplements to increase your immunity. Have some plenty of water and rest with balanced-diet. Avoid spicy and too hot meal while the ulcer is present.