MODIFY THE SEAT!!!
I have had horrible back problems for nearly 10 years including weeks where I couldn't stand up or walk. About 6 years ago I had the injections directly into the spine which reduced the inflammation to the point where I could live with it.
Every car that I drive is modified with a back insert that molds the seat to my exact perferences for comfort (including the race seat in my Spec Boxster). The easiest way to start is with towels. Buy a stack of the cheap ones at Walmart in differing sizes and start placing them where you need support. Once you get the towels at the right places and in the right thickness, then have an upholstery shop make an insert in the same shape and thickness.
Good luck and I (used to) feel your pain!