L.R.
You've seen a primary and you're going to see an internal medicine MD? And two dermatologists? I'm surprised because I found that only a dermatologist really knew how to treat this. I know -- I have seborrheic dermatitis too, and a dermatologist is much better at treating it, I've found. If you have it on the scalp you likely have it elsewhere even if you think you don't. Check with a dermatologist again and this time, take a list of every prescription shampoo or other med that you have tried.
Have you tried these meds/shampoos consistently over time, or did you try a few times, or for a week or two, and then stopped because they didn't seem to work? Some take longer, especially if the condition is serious.
You also may find that things like vinegar and essential oils (which can be very strong and can irritate skin even if diluted) are making things worse and not better -- by using those you might actually be irritating the scalp and stripping its oils, which causes the oil glands to pump out extra oils, which causes flaking and itching (I know, it's weird to me that extra oil means what seems to be a "drier" and flaky skin but it's true). I'd leave those things alone for now-- they can all be irritants.
Consider asking the doctor if you might need a prescription antihistamine. The itching etc. can partly be a histamine reaction, my derm. told me, and he put me on a nighttime-only antihistamine. (And no I'm not sneezing etc., he says the skin can need antihistamines too.) Ask what a derm. thinks of that for you.
Do tell the doctor that the itching is so bad that it's affecting your daily quality of life.
