For me it was probably something to do with posture. When I began to ride my posture was okay, what you would expect from a beginner. Then it got worse because I wouldn’t focus on it and I would focus on other things when i rode. When I began to ride a difficult horse that would rarely go, I wouldn’t focus on my posture at all because I was trying to get the horse to move! When they finally started to go I could focus on my position and I realized how based my posture was. I started to sit up straighter and taller and I used a saying that my dance instructors would tell us, which is “shoulder blades together”. This really brings my shoulders back and keeps me from hunching over. When I improved my posture many other habits I had changed and made me have a better position.
This was very long, more than you needed to know lol!
Anyways, I hope This answers your question!
I often close my eyes when I start a training, it allows me to feel all my horse's moves, and I feel more relaxed, so my legs are in the good place, I keep my back straight...
For jumping it helps too, I close my eyes a few strides before the fence so I feel the movement and I have the right position. I also did some training when I jump without holding my reins, it helped a lot!