Has anyone had spinal fusion at multiple levels to control instability?

I am 55 years old, female, small boned 5'4" 125 lbs. I am very active, have a horse and ride every day. I also am in constant pain due to spinal instability. I have been in physical therapy using the Muldowney method for 9 months and my spinal instability is getting worse. I have "twistings" in my spine (where one vertebra goes to the right and the one next to it twists like wringing out a washcloth, in the opposite direction) in my lumbar and thoracic areas. I am considering having a spinal fusion at all levels affected to correct the instability. My pain comes from the muscles "grabbing" to try to stabilize my spine causing incredible tightness, pain. spasms and immobility. Sometimes I can't bend forward to tie shoes or brush teeth without support. My posture is terrible but my spine is too unstable for me to hold myself in the correct posture, plus my spine is twisted out of place and it hurts to try to make it go the correct way. I am wondering if anyone out there has this type of problem and if you have had spinal fusion surgery with instrumentation with any success.

Thanks