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Ulcers such that you are describing can sometimes be caused by oral cancer, this is why your oral surgeon wants to remove it.
The most common type of mouth ulcer that is white (surrounded by red inflammation) and painful is called an "aphthous ulcer", also known as a canker sore. These can appear under or on the side of your tongue, inside the cheek or lips, almost anywhere inside the mouth. The exact cause is not known, but apparently they can arise from bacterial or viral infections, some irritation to the area, a weakened immune system, allergies, and a few other things including stress.
An aphthous ulcer will heal within a few weeks without treatment. The treatment for an aphthous ulcer is to keep your mouth clean, try not to further irritate the area so it can heal, and sometimes an ointment with a corticosteroid is prescribed. If you use Listerine to clean your mouth, be sure to rinse your mouth with regular water when you are done. The Listerine is irritating to mucous membranes in your mouth, and may further aggravate the ulcer is left on too long.
If your ulcer is no longer painful, most likely it is healing. It may be completely healed by the time of your surgery date. If it is (or will) heal, it is most likely not cancerous. However, it would be wise to follow up with your oral surgeon, whether it heals itself or not.