Try to practice with no stirrups it really helps! Also my instructor once told me "imagine that you have a 50 euro note on the saddle. It'll blow away unless you keep your butt in the saddle!" And for some reason this helped alot!😂
Make sure you are putting your weight in your heels and not piniching with your knees. If you have your weight in your heels then you should be able to sit in saddle. One way to help get your weight in your heels plus improve your balance is two point when you are flatting!