I know I will probably get the responses of "Wear a back brace." or "Don't know until you try it!". But here goes any way.
.............. It's been 24 years since I rode a horse and all my life the one thing I've wished for is to have a horse of my own to ride. My fiance, Dan, is dead set on making that dream a reality (I love you so much Babe!). But aside from being extra careful, getting an older more docile horse and wearing a back brace what would be some extra precautions I could take?
Well, I really hope it works out for you to have your own horse and to ride for the rest of your life (safely!).
There's no way I could make any reliable judgement on the safety/risk of your specific situation, but I can offer this observation in the case of my back problems....
I've never been through anything like your experience, but beginning about 15 years ago I had several occurrences of lumbar region low-back pain and although it had been oh, about 10 or 12 months since I had had an episode at the time that I began riding again (after some years away from horses) I was somewhat concerned that riding might aggravate my back or even cause an episode.
I am most delighted to report that after the very first trail ride, my back felt better than it had in a long time. And I quickly realized that this was not an isolated thing. The more I rode, the better my back was and the more certain I was that it was riding that was helping. Now, years after resuming riding, I have not had another "back incident" to this date.
I'm no expert, but I think that the motion that riding a horse at a walk causes in my back is really good for it. I go trail riding every time I can and really love it. I've seen no ill effects from the trot or lope or gallop or anything else either, I'm thankful to report.
I can't make any sort of recommendation as to the safety or danger of you riding, but in the interest of minimizing risk, I can offer this: all three of the times that I've fallen off a horse as an adult would not have happened if I had only been paying good attention. Your attention needs to be on your horse and you need to be very aware of where his attention is and what his state of mind is (as best as you can determine) at all times. If you "see the deer jump into the trail" at the same instant your horse does, you have a much better probability of keeping your seat if he spooks/spins/jumps/etc than if you're turned around talking to someone behind you when your horse sees the deer!
We can't eliminate the possibility of a fall, but we can sure do our best to minimize it as much as possible.
Also, if you want to take every opportunity to keep yourself in the saddle, I would recommend a western saddle. Having a nice, high swell to catch with my thigh during a "wheel around to avoid the man-eating deer" episode and having that horn to grab in an emergency has saved me more than once. I'm sure there are many/varied opinions on this point, but that's mine....
I'll pray that it works out for you! Honestly, it's a unique experience and there's nothing else like it....