I would at least get a back support to wear while riding if I were you. Sometimes it helps to stick an ice pack underneath.
When I ride horses that are uncomfortable I tend to ride in my two point position most of the time. It seems to hurt much less that way. Luckily back when I showed it was jumpers so that was acceptable. If I did need to trot I posted as much as possible but never went down as far as was typical when posting, sitting a trot was just asking for trouble.
Stretching before and after riding also helped a bit. Many of the yoga positions are ones that my physical therapist prescribed for me and they are easy to look up online of follow out of a book.