It was I who suggested scoliosis (my son has it). It is not fixable, sadly. The best you can hope for is that it does not advance further.
It shouldn't slow you athletically, though it might cause back aches. Didn't the doc give you some ideas on excersizes to keep pain at bay?
As for being balanced in the saddle I have two thoughts on that.
If you are overall balanced , I.e. 50% of your body weight goes down the left side of the horse and 50% the right, then you won't fall off and your horse can maintain its' balance, too. It's kind of like Jenga; the blocks might stick out on one side, but they are balance ultimately by blocks that stick out on the other side, arent they? If not, the tower falls.
That doesn't change the fact that you may have more pressure going into one side of the saddle than the other and that could hurt the horse. For that, one can fashion custom padding to the saddle seat . Problem is, that might through the upper body off and since you cannot bend the spine back, you would not be able to recover your balance.
Is you spine just above the hip fairly mobile and straight? That's where you need to do you most frequent and dramatic movement to absorb the horse's motion.
I know it's not much consolation to you, but all riders, scoliosis or none, are crooked . I notice that one one side my pant leg always hikes up and I have more horse dander on the back of my boot. That's me gripping up or being crooked. Alway have been, probably always will be.
Just think of what our horses put up with!
You read Sally Swift's book? She had scoliosis and had to do a ton of body work, The Alexander Technique . Heard of it? Look up Peggy Cummings, too. She is some bigtime body worker around here (WA) and I just heard about her today. I know nothing so will look her up myself, too.