I have a burn from putting cold sore medicine on my tongue. The burn is underneath and very painful what can I do?

Wait for healing. If it is really getting in your way, get a topical anesthetic compound, something like a denture sore mouth thing, from your pharmascist. You still have to wait about a week for healing.
Rinse saline. Take 1/2 tsp of salt to 1 cup water, warm and rinse 3-5 times daily to neutralize the burn and encourage healing . You can go to the drug store and get some orajel with benzocain and that will numb the area for a short time to give you some relief.