I am on full disability, have been since 2004. I could no longer work as I could not do the physical part of my job, and with the brain fog, I could not do the technical aspects either. I knew that I could not learn a new trade, and my flares would keep me from holding down any kind of regular work. I worked around firearms, and machinery in the shop, and I had become a safety hazard as well. They were all good reasons to apply for disability.
If you think you are too disabled to work (it sounds like you are), then definitely apply. I cannot imagine how sick I would be if I was still trying to work. Disability is there for people who need it. We paid into the system, and Lupus is a legitimate reason to apply.