I've seen so many posts here with all of your awful struggles with seborrheic dermatitis, that it makes me so sad.
While I too have seborrhheic dermatitis, I don't have it anywhere near as bad as you. In fact if you saw my face, you wouldnt even know I had it. I've always had a mild case of it, when I acquired it in late 2015 after a bout of depression. Believe me, almost know one has done as much research on pubmed than I have with the possible exception of a person's info that I'm about to post.
I just recently had a comprehensive stool test done and I DO NOT HAVE Candida Overgrowth or leaky gut syndrome. So while seborrhheic dermatitis MAY BE caused by Candida Overgrowth or leaky gut, IT IS DEFINITELY CAUSED BY A DIMINISHED SKIN BARRIER. You see, recent studies are coming out that indicate one of the factors in seborrhheic dermatitis is a diminished/malfunctioned skin barrier. By the way, all of those candida cleanses and anti fungal herbs might not even treat maleszzia furfur, but instead effect other yeasts. So to summarize, you need to first fix your immune system with kefir/cultured food EVERYDAY, then take diflucan in adequate dosage and duration, THEN rebuild your skin barrier and you WILL BE CURED.
Sebhorreic dermatitis is three fold problem.
An impaired skin barrier.
Dysfunctional immune system.
Fungal Infection of the skin.
There many different factors that can cause the three above but it is important to learn that it BEGINS and ONLY HAPPENS with 1. an impaired skin barrier. So... harsh topicals are incerdibly problematic (such as ketoconazole). Skin with a healthy barrier does not have .2 and 3.
But... it is easiest to eliminate .2 and .3 before taking care of .1
So... .3 the fungal infection of the skin can be eliminated or at least heavily tempered with oral antifungals like Fluconazole (several studies do show effifacy). Or topicals if you wish but good luck finding one that is gentle to the skin (many do report success with ACV and Coconut Oil... both are too harsh for parts of my skin so I do not use).
.2 Can be taken care of with Paleo-ish allergn free diet (gluten free defienitely, maybe dairy free), Vitamin D supplementation to sufficiency, adquete fatty acids (GLA and Fish Oil show good results in some) A & K2, Zinc, Iodine, Selenium and B vitamins (Biotin deficiency seems to be a huge player here... many anectodal cures of SD with high dose Biotin.. if you have been on many antibiotics for long periods of time you are probably very deficient as the intenstinal flora are the big source of Biotin). And of course, fermented foods and probiotics. Lots of anectodal reports around the web for these. Look into thyroid as well... many who have thyroid issues also have SD.
.1 This is the trickiest. Many report a complete clearance of dandruff/dermatitis after going no 'poo. Some only report good improvement, but no clearance. In these folks, the barrier is heavily damaged and needs some assistance to repair. There are several ideas for this... one is GLA supplementation (studies on oral and topical show clearance). Another is speficic products or oils/butters (such as shea or coco) that heal the skin barrier. Cetaphil Restoraderm has some glowing reviews, I came across a couple that claimed it cured their SD. Same with Cerave. Shea Butter has good anectodes too. Some peoples skin barrier is only midly damaged and heals with just simple water only washing.. others need some topical assitance.
My own personal SD improved with no poo... about 90% on the scalp and about 75% on the face. But a few spots remained... I went on a 8 week course of 200mg of Fluconazole and all the redness went away but the flaking remained... which leads me to believe that I still have a damaged skin barrier (most definitely true considering my topicals history) and/or a biotin deficiency (again, very probable looking at my antibiotic history). I am working on healing gut and a sub-clinical thyroid from VLC paleo and previous overtraining and undereating with cross country... my scalp dermatitis is 100% gone as we speak and my face is about 90-95% gone... all that remains is flaking. I am hesitant to try topicals for the barrier becaue previously topicals caused folliculitis for me (probably from the skin flaking off then clogging the pore)... but manually exfloliation helps.
I am currently about to try a 6 week course of 15 mins of dipping my face in a bowl of water mixed with Dead Sea Salt ... seems to repair the skin barrier.
I am 100% sure that this a curable condition and the cure involves all three of these mechanisms that must be addressed. The difficulty is finding the most effective methods to address the three for you individually.. as topical and supplementation response varies.