Go to any drug store or even Wal-Mart. At the tooth past section or tooth
pain reliefe section, you can find many kinds of product for "mouth canker
sore". It could be liquid, gel or cream. It can be used to relieve the pain
and help you eat normally and help the sore cure faster. Be carefule, some
are for lips (can not put into mouth) only. You need find those that can
be applied inside mouth.

Adding B12 and other vitamins into your diet should be done to treat the root cause.