use bailing twine and tie your stirrups to your girth. the bailing twine is strong, but will break in an emergency.
I have the same problem, my trainer has me do no stirrup work and I have definitely noticed a different
Use a gymnastic Ball to practise😄
Start by doing a two point and keeping your leg still at trot. Once you have that mastered add transitions while still in your two point.
Hope this helps!
Do some no stirrup work. This encourages your leg to stay still, as if your legs swing forwards or backwards, you will lose your balance and possibly fall off. Also, as well as doing flatworm, do some jumping without stirrups as well. Hope this helps!
Try doing small jumps without stirrups. Obviously being very careful as you do this. Not high jumps, but just little ones, and you'll learn how to keep your leg still without even knowing :)