You are right in helping your doctors try to figure out what skin disease you have. You must take an active role. Diagnosis is confounding because there are hundreds of different skin diseases and with each type there are sub-categories. Have you been tested for Allergic Contact Dermatitis, ACD? I am thinking of this because of your helpful photos showing the appearance of your symptoms and also your description of intense itching, both common symptoms with ACD.
You mentioned testing and I am unclear as to what skin testing you have had. Please be specific as I would like to know if you had the 4 or 5 Day Extended Patch Test? Many think they are tested for allergies with the simple 30 minute environmental allergy prick test which tests for environmentals like pollen, dogs, cats, mold, dust mites and such. The patch test is checking for elements, chemicals and even natural elements in fragrance or metal that are contacting you or off gassing into you in. It's checking for what you are touching, wearing, sleeping on, washing with and such and cause symptoms like you are exhibiting caused by allergies. This test requires 4 or 5 days with your back torso lined with patches and 3 days, normally Mon, Wed and Fri for doctor appointments that week, you will remember the nuisance factor of this test. My first patch test 5 years ago tested for 78 allergens and subsequent test panels this year for another 20. Google the 4 or 5 Day Patch Test and also the True Test, which is the test kit used on me, yet there are other test kits. An allergist did this for me, but most often dermatologists do this. Not all derms or allergists are licensed to perform this test, so ask your doctor to refer you to doctors who are. Also your Family Practice doctor can find one who is or you can even contact the patch test companies for doctors in your area.
Out of all the patch test sites I tested positive to about 7 and learned what to detox from my world and this has given me my life back.
Also your symptoms can look like those with Atopic Eczema, AE. The causes of this are confounding to me and this is common in young people, some are even born with it. In the last year or so a new drug Dupixent has been helpful for many, it does have neg side effects for at least 10%, but it is miraculous for many. It is wildly expensive. Check this out too. AE is probably the most common form of eczema.
You are in a discovery stage, please also go to the Inspire Website and look into the eczema skin site. I blog on this site and in my early days with eczema learned a tremendous amount. I believe I know as much about my eczema type/types as my specialists do now. I still blog on it and just this week a friend guided me to a new treatment. On Inspire there are lots of members with Atopic to answer your questions. On the Mayo site in March I learned about a natural herb to help with one of my rare forms, a form which no prescriptions gives relief for and this "crackpot" idea, drinking cilantro smoothies daily worked for me as it does for about 50% of us that try it with Transient Acantholytic Dermatitis, TAD aka Grover's Disease, GD. Eczema and dermatitis are words used interchangeably. Amazing what we can learn on the internet.
Skin disease diagnosis is challenging and another very helpful site, not for blogging but for learning for people like me not in the medical field is dermnetnz.org. Best compilation of every type of skin disease, not just eczema which I have, but other skin diseases. Actual photos of fellow suffers for you to compare your physical symptoms to, along with a list of symptoms with each disease. I began with ACD at age 50 and now at age 70 have 2 additional forms. This is common with eczema which is an auto-immune disease and if you have one auto-immune disease, especially at age 20,you are likely to develop more. The New Zealand web site I mentioned helped me guide my doctor to figure out my newest form of eczema. She was treated it incorrectly as one of my previous forms and because of my study on this site looking into my specific symptoms and the photos I found the name of what I really had. The steroids she was using to treat it actually fueled my newest form, it fed it. I took info into her and she agreed and changed treatment. Also understand some symptoms will have papuales or pustules which you have in your rash that are larger/smaller, more or less extensive and slightly different,;eczema is not exact. Location on the body is often a clue.
I know this is lots to take in and perhaps you have already had the 4/5 Day Patch Test, but keep up blogging and asking others for help to get the name of your tormentor because we understand your misery and want to help. I was to the point of suicide 5 years ago and today, even with the of addition of 2 more forms of eczema I am almost symptom free and getting a good nights sleep free from intense itching and pain and back to living a normal (but controlled) happy and productive life. The key is personal research and good doctors (I have dealt with over 6 derms and 1 allergist and learn something new from each one).
Let us know of your progress, we do care.