What does your dermatologist think that it is? I've always been prone to "unexplained" rashes, but nothing like that and it does certainly sound like a fungal/yeast type rash, especially given that it gets worse when your blood sugars are elevated. Have you ever tried an anti-yeast dietary regimen?
Something I've always had a problem with is redness, flaking and itching near my eyebrows and around my nose. My current doctor finally said, you know that's seborrheic dermatitis and we have a cream that will clear that up in a hurry. It's called ketoconozole cream and it worked like a charm. It might not be advisable near your mouth, though, since it's quite toxic if you ingest it and you also need a prescription for it.
I'd still suggest working with your dermatologist to figure out what this is and then find the best treatment for it.