How do i prevent this recurring ulcer underneath tongue?

First , see your : Dentist for a diagnosis of this lesion. Ulceration under the tongue is most likely from trauma. Trauma from food (abrasive, sharp, chemical) or from the lower teeth when the tongue extends over the teeth. However, there are other causes that may need to be figured out. Please let your dentist take a look!

Recurring ulcer: See a dentist to get it examined and diagnosed. The diagnosis will determine whether it's something that can be prevented, requires treatment, or removal and biopsy.