I'm a western rider, mostly. And I mostly agree with
although there isn't a lot to be done about it.
"He is big and really pushy, so the owner has me keep my reins tense.
That was what I tried learning to ride on Mia. Try to control her through the reins. She soon figured out how to ignore the snaffle entirely, any time she wanted. It turned her from a sometimes bolter into a confirmed, frequent bolter. Fixing that took some serous work and risk while doing it. Larry Trocha, a western trainer, says horses always get duller in the mouth with time. I like Larry Trocha, but I find that view tragic. Horses get duller in the mouth due to too much pressure, held too long and without hope of release. Horses should get SOFTER with time. MORE responsive. If they don't, we are ruining them
. As many lesson horses ARE ruined.
"it's because I'm not aggressive enough with horses
That is horrible terminology. Please replace "aggressive" with "directive and gently persistent". I flew in two-seat fighters in the Air Force. As a newbie, they told me I needed to be more aggressive. But "aggression" is not what they REALLY wanted. They wanted me to be more confident and willing to provide direction.
The same is true with horses. Knowing what you want and how to communicate that to the horse is a good thing. "Quiet Persistence" is how we QUIETLY teach the horse. "Aggressive" needs to be tossed out of our vocabulary! Denny Emerson is a truly great competitive rider with a lifetime of achievement. He wrote:
"So, how should I have obtained the response? He didn't listen to my light leg pressure. Why shouldn't I use more if he fails to respond? Well, because of what I just said. Harder and harder pressure makes him more nervous. The way to get him to move is not to bang on his sides, but to 'pester' him until he moves.
I have never read any book about riding that talks about 'pestering' a horse. They always use the terms like 'Apply the aids,' as if the horse will magically understand what the heck that means.
But think. Why does a horse swish his tail at a fly? Because the horse knows the fly will bite him if he doesn't get rid of the damn thing. The horse is not terrified by the fly. The fly isn't an attacking mountain lion. It isn't even a stinging wasp. It's a fly. Even an annoying fly will get a horse to respond, but it is not a panicked response.
In a way, we riders need to be mildly annoying flies..." - Denny Emerson, Know Better to Do Better, Mistakes I Made With Horses (So You Don't Have To)
Great book for less than $15 on Amazon. Tough to do with school horses though because school horses are perpetually ridden poorly. All their riders are learning to ride. It is expected. Also sad, because it teaches students to ride badly, using more pressure, more punishment and "aggression" instead of giving helpful advice to the horse and quietly persisting until the horse yields to the "mildly annoying fly". Now, your question about how to post with quiet hands
: Hands move out of synch with the horse because the rider's body is out of synch. Ride in two point if your instructor will let you. Stand in the stirrups with plenty of contact between your leg and horse, just off his back. Use flexing in your joints to absorb most of the up down shock of the trot while staying steady. Take hold of the mane with one hand if need be until you get used to the balance
in two point.
Once you can ride balanced in two point, your hands will easily stay where you want them. Like an unstable leg, hands that are not steady are a symptom, not the problem!
Once you learn the balance in two point, KEEP that balance - balance is balance after all - keep it and merely fold and unfold your body when posting. If your body stays balanced over the stirrups as you rise and descend - not fall, descend - then there is no change in balance forcing your lower leg or hands to move around.
How to Test Correctness of Rider's Position: If the rider is in balance as a result of his upper body's being properly inclined forward, he is able at the walk, trot or gallop, WITHOUT FIRST LEANING FARTHER FORWARD and without pulling on the reins, to stand in his stirrups with all his weight in his depressed heels.
In executing this exercise the seat is raised just clear of the saddle by stiffening the knees but keep them partly flexed. The upper body REMAINS inclined forward at the hips. At the trot on hand should touch the horse's neck LIGHTLY to assist in remaining in balance. At the walk or gallop [canter] the rider, if his seat is correct, should be able to stand in his stirrups without the aid of his hand. A rider, who can execute the above exercise at all gaits and without first changing inclination, is in balance and never "behind his horse". The majority of those NOT in this position partly maintain their balance by hanging on to the reins, thus unnecessarily punishing their horses' mouths as well as their backs." - US Cavalry Manual
If you sit on the horse and use his effort to thrust you up and forward, then your balance is changing dramatically. Since your balance is changing, the hands and legs move around. We then try to stop that movement with muscular tension or gripping. That can hide the symptom but doesn't solve the problem.
Two point teaches you to keep your center of gravity aligned with your horse's - and your horse shifts his center of gravity when turning, slowing or accelerating - so two point while he does those things teaches you to adjust your balance to stay "with your horse". If you stay with your horse thru the motion of posting, your legs will be stable and your hands won't need to move around - because your balance isn't moving.
That said, I don't know if your instructor will let you work this way. I'm also not an instructor, competitor, teacher, etc. Just a backyard rider.
And every book I've got discusses posting as requiring the horse to push you forward and up - which strongly implies you go from "behind your horse" to "with your horse" back to "behind your horse" every couple of strides. No one teaches posting as staying in balance at all times and merely folding and unfolding your body. But when I manage to do that, it works for me. "Works" as in my hands and leg are stable because my balance is. And no, I don't find it easy to stay "with my horse" while riding. But when I fail, it is revealed by unstable legs and hands. Fixing my balance fixes my legs and hands.