Originally Posted by MissingStar
If I feel comfortable in the saddle, that's when I know I'm sitting wonky. I have to shift my weight back to centre. This then makes one leg slightly shorter than the other because of my tilted pelvis.
I also have scoliosis, and I know this feeling well! I have ridden with a biomechanics specialist a couple of times, and she and I worked together to find the right thickness of shims for my stirrup and under my seat bone (using a shim between the seat of the saddle and a Seat Saver, so as not to affect the fit of the saddle for the horse) to make straight feel a little more straight. It's still tough to keep everything in alignment, and it does get very painful after a while, but it helps.
That being said, shimming is not something I would play around with without a very experienced set of eyes on the ground to evaluate it. The first time I rode with a shim added to my stirrup, I thought it was a huge mistake, and that it was making me twist terribly and making my leg swing forward. Nope, it just felt awful because it was correct!
I would also encourage you to have the flocking of your saddle evaluated more regularly than otherwise might be necessary. If you have a tendency to weight one seat bone more heavily than the other, you will likely compress the flocking unevenly and end up with a wonky, crooked fit. That could be contributing to your horse's unwillingness to pick up a particular lead, combined with your own crookedness.