Hello and thank you for your question.
This sounds like you have the beginning phases of an aphthous ulcer. The white portion is a membrane. The best thing to do for this if an ulcer is to break up the membrane, get it to bleed and allow exposure to oxygen
Recommendation is to apply oral gel, take your toothbrush and poke the membrane until you have it bleeding, then dry the surface and take a cotton tip with hydrogen peroxide
(no higher than 3% per volume) and apply it to the surface of the wound. This will hurt, I will warn you. But you will feel much better afterwards and it will heal a lot faster. Aphthous ulcers
will take 1-2 weeks to heal.
This will also help: avoid spicy foods, rinse your mouth with salt water to help shrink the lesion (take 10 ounces of warm water with a teaspoon of table salt, swish and spit the entire glass 4-6 times a day especially before and after meals), take anti-inflammatories such as ibuprofen
I hope this helps, best wishes and health