Ooooh, this is a good question. Not good that your horse is sore, but it’s something that everybody needs to spend some time thinking about.
Can you ride with an experienced endurance or CTR rider or two and see if they can pinpoint something for you? Or get some good video of yourself schooling your horse out in a field, then possibly post it here?
I switched to treeless, because I worked with 2 saddle fitters, and probably 10 different saddles, which all supposedly fit “beautifully” and my horse was still crabby, posty legged going down hills, and developed white patches behind the withers. Before treeless, my horse had no trot at all. She wanted to walk, jig, or canter, and gallop was even better. Since treeless, she can trot out like a power house. The tree, no matter how well fit, was impinging her scapula.
I am a HUGE believer in treeless now. I see so many of them at endurance rides and always ask people about them. People always tell me that they feel trees can never be fit “perfectly” to a back, because the back is so dynamic. Constantly flexing, changing, arching, dipping, and twisting with movement.
And another problem with custom fit trees is that the horse can gain or lose 50 lbs here or there with the seasons, or with muscling, and then does the saddle still fit? In my case, it did not. Maybe specialized saddles are different though? I know that they’re supposed to be more advanced than a standard tree.
If a good treed saddle is working for a horse and rider, then there’s no reason at all to change. Trees aren’t “the devil” but I do believe that for certain body types, you will never find a tree that works for that horse for the long haul.
For my particular style of riding, and my horses, I feel I made the best decision possible in switching to treeless. At this point, I’m sure I could never go back. I’ve ridden a short ride here or there in an Abetta or Aussie, and not only do I feel very uncomfortable, but I can tell a huge difference in the horses. They just don’t have that totally un-inhibited, freedom of movement and you never feel like you can effectively communicate with the horse. But that’s just “me.” Treeless is not for everyone. 