Don;t worry so much about cancker sores. You can go to doctor than they will do a lot of test on you to find out what is causing it to really find out cure. Otherwise, just treat the symptoms with orajel.
The worst case can be is it can cancer if it is really not going away. I hope I am not scaring you.
And are you really sure it is cancer sores ? It can be other things so better get to an physicist.