Hi Lily,
Have you ever considered or tried an elimination diet to see if a particular type of food was causing the dermatitis? Sugar, gluten, and dairy can wreak havoc on your system, causing major dermatological issues such as you've mentioned with your ulcerations. If you're not familiar with an elimination diet, there are books and websites on how to try it. Also, a nutritionist can help as well. It takes about 4-6 weeks of totally eliminating the suspected food, then reintroduce it back into the diet. Your skin will clear up if there is an allergen or sensitivity, and you avoid that particular food. It takes some strict following and patience but is very reliable.
I understand how frustrating it must be to be unable to find a solution, so I really hope you find your answer soon!
Keep us posted if you figure it out! Sending best wishes for healing!