Yeah!!!! The fear is always there - it lessens with time, but it is still there... Hbo as far as I know is very expensive and very disruptive (an hour or two daily for 20 days?) for a mouth ulcer. It sounds a little like overkill.
I had a Nasty ass ulcer that became an issue for about three weeks and this was almost a year after radiation - I think it was a hold over to be honest. It may have been there from the end of radiation but untreated it got deeper more it irritated, and painful etc. Anyway it looked like it was either where one of the rad beams entered or exited my mouth./tongue (There was a matching burn area the same size and shape across from it on my lower cheek - internal - I actually was purplish. Anyway my ENT prescribed oracort - directions said use twice a day - that helped only minutely - so then I made a habit of keeping the area super clean, after every meal or drink I rinsed or cleaned my mouth then applied the oracort - within three weeks it was gone. hopefully this will heal quickly for you.. Do try to keep the area clean and minimize irritation to the area if you can. Hugs