Hello, I understand your problem. This is not very common problematic the diabetics but once it has come it will last for long because of the sugar levels in the blood will delay healing. More over the habituated bacteria of oral cavity are unharmful nut the other bacteria which grows along will delay. Healing if the ulcer. You may take multi vitamin if it is.normal ulcer if it is not taking too long to heal then it need medical intervention. Thank you