thanks for your query,
the features you are presenting with suggests its a chronic ulcerative lesion,
it could be recurrent aphthous ulcer, or recurrent herpetic gingivostomatitis
(viral infection) or pemphigus vulgaris
), to rule out all these things you need to undergo biopsy means surgical removal of a small amount of the tissue and examine... for time being you can take topical steroids like triamcinolone acetonide
0.1% 3-4times daily for 5 days....once the right cause is found out we can start with a narrow treatment..consult a oralphysician for investigations..