It is not advised to inject hydrocortisone. I think you might have misheard your doctor. I think he might have just mentioned cortisone injections which are administered to joints as well to reduce inflammation.
I had cortisone injections and they did not work for me. The last biopsy I had came back negative for microbes but revealed hyperkeratosis, which is severe inflammation. This is why the doctor thought that cortisone shots would help.
The best thing working for me right now is the excessive mouth hygiene method combined with the Bepanthen cream.
One thing I was also trying was actually getting a high level dose of vitamin E softgels, making a small hole in one, and then squeezing a small amount of the contents of the softgel onto my lips, which would probably be the most potent type of vitamin E available in a liquid form & with minimal additives.
I remember reading how one person had great results--even got cured--from doing that with aloe vera softgels.
I am steering clear of products with petrolatum in them as they just made my buildup appear discolored, turn soft and then fall off quicker during the week or while in the shower.