
I'd be very very surprised if Sidesaddle is any better for your back - it means you only have the support of one leg as far as shock absorption goes, rather than two when riding astride, and if you don't get your position absolutely spot on, it will cause twisting and uneven strain down your back.
It IS better for people who find riding astride hurts their hips/pelvis though.
I would suggest you book yourself a half hour lesson with someone who teaches sidesaddle, and see how you get on, but - even if it does suit, firstly not all horses will take to sidesaddle, and secondly, finding a sidesaddle and getting it to fit your horse is going to be something of a nightmare.
If you really can't ride any more, have you thought about taking up driving, which is something your horse ought be able to do!