I'd rather see someone leaning back (not too far, but having a deep seat) rather than slumping forwards.
*sigh* back problems seem synonomous with horseback riding; some days I feel like I'm twice my age!
Aawww ya poor thing. Back pain is no good. I was born with bad back problems and battle against them in many ways. Regular chiro visits are a start and eating more ginger in your diet ;) also, talk to a doc about an anti inflammatory. This is what I take when things get bad. It relaxes the muscles and helps with any painful inflammation.
Its hard to manage and sometimes it seems unbearable bt I've learnt to live with it by keeping p with maintenance on my back. Unfortunately when we have horses sometimes we spend so much time making sure they are comfortable that we ignore ourselves.
Good luck with it all
Oh and a warm bath with epsom salts and lavender does wonders ;)