seborrheic dermatitis UEGENT PLEASE READ

What is the root cause ????? Around 4 years ago I started to have bad IBS , at around the same time I developed seborrheic dermatitis. I was given steroid cream that worked at 1st but came back on occasions. Both IBS and seborrheic dermatitis continued on for months after . This was stressing me out any I took time from work because of this. At first I thought the seborrheic dermatitis was caused by a tanning product or hair dye I was using , I came off them but on occasions I still had seborrheic dermatitis. It cleared from my scalp but my beard and nose area was still effected At this point I was working out and weighed around 14 stone . I read a lot online mentioning it was due to diet so I tried cutting many thing out and to this day I don't have milk, cheese , egg ect. At this point cutting foods out I went to 12 stone so this really effected me My seborrheic dermatitis flared up when I got married and effected me on my honey moon going in the sea as that would make burn and become increasingly red. I've been to the hospital for tests and I'm only allergic to dust mite. I tried all different creams /oils eg coconut oil for my seborrheic dermatitis. I then started to try be less stressed and it went away well cleared up for a while and I went back to 14 stone eating and drinking fatty foods still no dairy. My son was born July this year and the build up to that my seborrheic dermatitis has been very bad and I have tried many different products to help eliminate the redness and flakes. I then tried ACV this helped on my face well around the nose but not in my beard. Now it comes and goes day by day. I have a stressful job , I was my hair and face daily with aquaes cream and simples light moisturiser. ACV soaking daily on the face but there must be a cure?????? My face feels dry by the time I finish work and shower with cold water soon as I'm home. I use head and shoulders itchy scalp shampoo/conditioner and this helps my head. I have just started to take 1000mg Biotin as I've read this helps. I have grown and beard and over the last 6 months this has been fine but now I have bad beard dandruff and redness . What I fully want to know what causes this horrific mess: -stress -bad gut and I have IBS from time to time - diet / fats suger dairy HELP