Rye, I agree that focus on the horses hind end solves a lot, including a lot of bridle and straightness issues. You need enough forward to funnel the horse, and I find that the more I work on myself, the easier it is to listen to the horse. I think where people get into trouble is that they forget that the horse still has to follow the rein and yield to it, or conversely, they make everything a rein issue. But, in the horses that are behind the leg, I find most of the issues are solved simply by getting the horse to go forward, and then to Chisamba's point, making sure that it's an "honest" forward, and that you don't have to beg or kick every stride.
Just to further the conversation, and I sincerely don't mean to have hijacked this thread, but I had a fantastic lesson where we talked through these feelings. I was on a horse, who while she's very highly trained, it's not correct - she flips leads from the spur, they're not really controllable, she doesn't have gears in the gaits, and as a result, likes to boom around in a big leg flinging trot or flat, stiff canter. I've been getting good feels from her, and since I'm able to "feel" the progression in training, it's been very educational. In any case, we worked on gears - going from soft jello to egg in the trot (thinking passage up the trot to make it a bit smaller for a few strides, and then soft to let it back out), and in the canter, we worked on the same - but from canter on the spot to regular canter, trying to feel the threshold in my body since she has a hair trigger with the collections. Too much egg (tense up too much) and you get the playground horse on the single spring feeling - this thing: https://www.playdale.co.uk/playground/spring-horse.html
I didn't have much trouble with that - but when we went to changes, I collected the canter too much - allowing her to freeze and flip. So I needed to be softer in my body. I got the canter, but then the changes them selves kept flipping harshly over, as if she froze and then switched legs as quickly as she could. What I realized when I complained about my seat being hard on her back, what that MY seat was hard!!! I was tensing every muscle as I was changing. I don't know why I didn't feel that before. I had gone from doing a set up from the change which should have been very subtle to going hard throughout my body, rather than keeping my butt and thigh soft - that damn egg again!! So, I tried my best to just do a tiny set up, but keep everything (as Andrea admonished) in slow motion and soft - and low and behold, her changes changed! It was incredibly eye opening!
Right after that, I rode Gala, and when I got to the changes, I started my same MO, getting hard throughout my body like an iron rod. The second that i stayed soft, and didn't SET UP with CAPITAL LETTERS, the changes were so soft, and so easy and right on my timing. I could have cried it was such a different feeling. I wish that I had figured that out a year ago, six months ago. So, from here on out. Egg and Jello it is... or whatever analogy works. The key for me is that I engage way, way down between my pelvis wings which enables the rest of my body to have a softer feel. Then I can isolate - do I need a bit more thigh for support, is the horse coming into my hand so I need to firm my back up to resist it's lean. It's very nuanced and it's going to take a while to stop forgetting to not stiffen my whole damn body.