I like to squeeze my bum together and put pressure on the saddle with my knees, not my legs 😊
If you have a good horse that isn't too sensitive, you can feel for the girth with your toes. Stirrupless and bareback ridinng are both really good for this too because they open your hip and align you.
Try having a super straight back and your legs almost always move themselv back
Try to squeeze in maybe I have the same problem