How do you best train your balance?
To be able to sit properly on the horse requires a certain balance.
It can be innate but you can also train it just like everything else.
Good training is of course to ride a lot without a saddle, but you can also sit on a big ball and practice sitting when you lift your feet.
One thing I practiced a lot when I was young was to carry a book on my head. It’s a bit fun and you can compete to see who can go the furthest and keep the book on your head. I got so far that I could lie down and get up again without losing the book. Try it! It also helps your posture, as you have to walk straight to be able to perform the exercise.