It really sounds like a cold sore, which is a form of herpes. although I am impressed your doctor didn't already know this. Does it always come to the same area of your lip? Ask your doctor about cold sores. If it's that, then you need a prescription of Valtrex or Zovirax to keep them from showing up.
Also, don't be embarrassed. 95% of the world population is exposed to herpes by the time they are 18, and 30% of them get cold sores. Annoying and ugly, but very common.
Its worse than cold sores and it some what allouded the docs as they have taken skin scrapping and sent them away to the lab to see if there is a fungal connection to it as it clears and then returns a couple of weeks later.
I was on a course of very strong tablet and they helped a bit and then when the course was completed the sores returned with avengance
A fungus wouldn't have blisters like that. Have you tried getting a second opinion from another doctor? I would try going on a low daily dosage of Valtrex to see if it keeps it from coming. Good luck