I hope the warm bath helped, and the swimming too. I'm sorry you're in so much pain - you're probably right that you need to accept that you'll always have it, but that doesn't mean you can't get the best help available. I've been wondering what sort of pain relief you've been offered. We all find different combinations work for us, but it's worth trying a lot of different things to get the best outcome you can.
Have your doctors connected the vasculitis and/or the neuropathic pain with SjS? Do you see a neurologist? I also wonder whether they've considered treating you more 'aggressively' ie with immunosuppressants?
I had a sudden period of back pain about a year and a half ago. Like you, I'd never had back pain before and couldn't believe how awful it was. It turned out to be osteoporis - probably connected with both SjS and long-term Prednisolone treatment - and I had a series of small fractures resulting in a loss of 3" in height in a couple of months. I learned a lot from this, mostly to take severe pain seriously and not to assume that everything was a 'normal' aspect of SjS.
Hope the swimming was fun.
Thinking of you - Chickpea