28 here. Hadn't ridden in a decade and only started again recently. Younger me could jump but current me is clumsy as hell. I only have one bit of practical advice and some on fear and behaviour. I worked with a lot of dangerous & wild animals in my time. Big cats, primates, bears, crocodiles and other beasts out on a safari range or on a film set. I've learned "three types of trust":
1. trust the animal, their nature and/or in their training = leap of faith
2. trust the professionals around me to keep me safe
3. trust in my own ability to assess, prevent and react to a variety of dire situations
When learning, we usually don't have the skills for number 3 and I think that's what is holding you back and giving you anxiety. You either need to acquire the knowledge for number 3 OR you need to actively push yourself to go with number 1 & 2. When I cantered for the first time in the woods I was so scared of bolting that I nearly puked. My instructor said "don't worry, he's a hunts horse. He loves being outside so will go a little faster than normal but he will look after you. Ask him to slow and he'll slow, I promise." She asked me to trust her, trust in her horses training and trust in his experience. I couldn't trust myself as I was being irrationally fearful given the situation, so I gave a little more rein to avoid giving conflicted signals and asked him to canter. It was GREAT. But no lie, it takes courage to face our fear. And when we're scared, even if it's a fear of failure, then it's hard to learn.
When typing a sentence do you think "I'm pressing the letter I, then I'm pressing space bar, then I'm pressing caps lock to release the capital letter". Nope, we just type out the damned sentence. How? Because typing is muscle memory and it's an action in our learned subconscious mind. Like driving, like walking, like posting :) Once you can post comfortably it'll go in your subconscious. Once you can hold your reins properly it'll go in your subconscious. And once they are in your subconscious you would now have space to focus on other things. Y'know, some lessons I'm having I'm just purely focusing on my heel. Not my whole leg, just my heel. Not my posting, but my heel when posting. Or sometimes just picking up the correct diagonal in the first place. Once that habit is in my subconscious I'll move onto other things. Piece by piece it will come together.
Question: do you understand how horses are trained? "To learn to write you must first learn how to hold a pen. As you write, you learn to read." We give the horse a pen and show them how to hold it (teach how to give to pressure) and then we teach them how to write. And they write a million stories. About how they learned to move their hips or give their head or stand still or wore a saddle or took weight on their back. Then, once they know how to write very well we introduce a rider. We show the rider how to hold a pen. And then we teach the rider how to write. And guess what? The horse knows how to write too! And soon enough they are sharing and writing and reading stories in the SAME language. As the horse and rider progress the language becomes more complex. It's ok, my German is probably better than my horse writing! There is no way to cheat it just takes time. Youtube some training videos about how a horse learns the necessary skills to be a riding horse. Learn their behaviour and try to understand them better beyond just the rising trot!
What do you need to know to feel less anxious while mounted and to get some confidence back? Maybe learn three different ways to stop a bolting horse? Are you afraid of falling? Strange, since you're willing to stand next to a horse, so it can't be that. Hm. :)
Anyway practical advice for getting the post in your subconcious: buy a swiss exercise ball on amazon. sit on it as if on a saddle and just like a horse let the bounce propel you up and use your legs to touch back down gently. Then watch a few videos of people riding and trot in rhythm with the rider as it'll usually be faster. PS far from easy. Then practice it with your hands in the correct position. Try to be relaxed about it. Be straight but not tense, be fluid but not flappy. It looks stupid but I did it and I made my beginner friend do it lol. And you know what? I bet you can bloody post on a swiss ball so you sure as hell can post on a horse! Honestly, check out the exercises on swiss balls just for the benefit of riding (youtube).
You got this! xx