While I have not had this particular type of cancer, I have had recurring neuroendocrine tumors (NETs), which is a rare form of cancer. To date, I've had three surgeries.
I'm inclined to agree with Lisa, @lisalucier, that you would do well to seek out a second opinion in this matter. The mouth sores sound very uncomfortable. Do they affect your ability to eat? If so, this is affecting your quality of life as well.
Recurring or metastasizing cancers are difficult both physically and emotionally. How are you doing with these challenges? Do you have a good support group in place with family, friends and other community resources (like a place of worship or community activity group)?