There’s probably only a 1% chance this is relevant, but... in 1979 I got a cold sore from kissing someone, but either it was a particularly virulent strain of the virus, or it fond me a particularly suitable host... anyway, it quickly spread into my mouth and throat where I developed large numbers of cold sores. I was in absolute agony for a week or more – could hardly eat or drink etc.
Eventually it subsided, but flared up again for years. Now it never flares up externally, but every so often it does INTERNALLY, giving me really nasty sharp, stabbing pains in OTHER parts of my head, like my eyes, cheeks, forehead, gums etc – never anything visible This can last for a couple of weeks before it goes, and is really, really horrible.
This happens because the virus retracts along the nerve fibres, towards the brain and spinal column, and lurks there – then when something (largely unknown, but for me stress definitely plays a part) triggers it, it creeps back down the nerves towards the skin, where it causes intense pain.
I don't know if there’s any possibility you’ve ever had shingles or anything like that – like I say, just a tiny chance maybe it’s not diabetes-related neuropathy as other things can cause it too...