Can you tell me what is that white ulcer under my tongue?

Hi doctor,

I had a minor accident when I was around 11 and that led part of the crown of the front tooth chipped off. The abscess started forming under the tooth in my 20s. But, it was occasional and did not hurt. I would just put some pressure on my gums and it will come out. I had undergone root canal treatment around two years ago, but that did not stop abscess from forming. I had been lazy going to the dentist again. At last, my front tooth got removed a couple of days ago. During the procedure, the abscess was removed, bone grafting added and the gum stitched. But, there is a large white ulcer being formed under the tongue after the removal. My dentist said that it is just dead skin and will heal in a couple of days. But, I do not see any improvement and it hurts. I am on antibiotics for five days. Can you check what that white ulcer under my tongue is? When will it heal?