FWIW, I think dressage is oversold as a style of riding. The books I have suggest it is for everyone. However, the dressage seat often taught is the one used by athletic, skilled riders on highly trained horses to demonstrate the skill and training of both. The dressage oriented books I have downplay how hard it is to ride dressage well, and by doing so they harm the average rider and horse.
For example, the modern dressage seat has a vertical line from ear to shoulder to hip to heel. The foot will be mostly flat in the stirrup instead of heel down. In comparison, the traditional western seat (which looks to me like most of the paintings and statues of riders going back 2500 years) has the leg angling to the front of the rider.
I've been playing around with this for some time, but riding 'western' in an English saddle has clarified for me WHY I find western easier: it uses my joints to my advantage. For example, most books, including dressage books, speak of having a relaxed leg draped around your horse. You should not grip with the knee, but let the weight flow uninterrupted into the heel.
Using the traditional western seat, I can adjust my stirrup length so that the bend of my knee is at the widest point of my mare's barrel. It doesn't matter if my hips are tight. It doesn't matter if some of my muscles need to stretch during the first 15-20 minutes of riding. It allows a joint - my knee - to follow the shape of my horse and drape around her in a relaxed manner. In this position, it is very hard to grip with the knee because the knee is folded around the shape of the horse. To start gripping with the knee, I need to bring my heel back.
Also, why does the cinch go around the horse where it does? It is because that is the smallest circumference of the horse. That makes it the easiest place to wrap my legs around her, because it is where my legs need to spread the least. (This isn't a contradiction...I want my legs to go where her barrel is smallest, and then adjust my stirrups so my knees lie against the widest point on her smallest circumference).
This vase is from roughly 540 BC:
This is an illustration from a dressage text written in 1729:
That has been the easiest place to put your leg for a LONG time!
It also has some disadvantages. I'm told by much more experienced riders that it limits the cues you can give with your spurs, and that you can give a greater variety and more subtle signals to your horse if your heel is under your hip. And I believe them. I don't ride with spurs and don't give my horses lots of subtle cues with my heel. My horses are not highly trained, and neither am I.
For a good, experienced dressage rider riding a good, well trained horse, there may be a lot of advantages to having the heel under the hip because of the cues you can give and the subtlety and just how impressive it looks. And I'm sure a well-trained, experienced rider who is a competitive athlete can make it work without gripping with their knees.
But I'm a guy who has done more weightlifting than dancing, and started at 50, and who spent 40 years jogging daily. I have tight hips, tight legs, and I've spent 5 years riding struggling to loosen my knees and get weight into my heels. When I move my heels about 4-6 inches in front of my belt buckle, and adjust the length of my stirrups to put the fold of my knee at my horse's widest point, it becomes hard to grip with my knee. It becomes natural to let my leg fold relaxed around her, because I'm letting the joints do the bending.
The same is true of moving with my horse's back during a trot or canter. In the traditional western seat, the place most humans bend easily - the waist - is used to absorb the motion of the canter:
You follow the motion of the horse by moving your hips up and forward by unfolding at the waist. This is easy. It will NOT impress a judge, but it is an easy way to move with your horse. I tried it earlier this week on Mia in my English jump saddle, and it worked. For the first time since I learned to canter a couple of years ago, I was able to keep my rump in contact with the saddle while moving with her well enough that she stayed happy and moving. The only change I saw was that she shifted some of her weight to the rear (following my weight), and thus cantered slower than when we canter in a forward seat.
The spine is not meant to compress vertically. A lot of threads discuss how strong and flexibly you must be, working from your core, to do so. And the end result, with a skilled rider on a good horse, is darn impressive.
But I ride 3-4 hours/week tops. VS Littauer argued that recreational riders who rode less than 6 hours a week would not have the physical conditioning to ride like a more frequent rider, and that they (we!) needed to adapt our style to stay within our limitations so we could ride without irritating or hurting the horse. I think that is true. It is as silly for me to think I can ride like a top rider while riding 3 hours/week as it is for me to think I can run like a top runner while training 3 hours/week. Or play football like a pro football player while only practicing 3 hours/week.
And as an older rider, security is important to me. Staying in the saddle, moving with my horse, keeping maximum contact with her - those help me stay on. Pretending I'm a competitive dressage or jump rider does not. My legs will not drape around my horse at my horse's widest point, even if she is an Arabian! Instead, that creates tension, causes my knees to grip, pushes me up out of the saddle, makes it hard for me to move with her...lots of bad things happen. And they will continue to happen, because I don't ride enough or have years of training in riding that style.
Just something to think about. I think it is fun to watch dressage videos. I also like to watch folks jumping 6 foot oxers, but I'll be darned if I am going to try it!
And if any of this doesn't help, feel free to ignore it. I'm a total nobody in the world of riding.