Get OralMedic on Amazon. It'll cauterize it immediately and stop the pain. While you're waiting, you can try to numb it with something like Kanka or OralGel.

https://www.amazon.com/Oralmedic-Mouth-Ulcer-Treatment-Healthcenter/dp/B0015X5ZW4/ref=sr_1_2?dchild=1&keywords=oral+medic&qid=1628022209&sr=8-2

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