Try rising trot with no stirrups
No stirrups is great for this as is 2 point at the walk/trot.
Tying your leathers to the saddle may cause issues with retrieving your stirrup or hanging up if you fall. I wouldn’t suggest that method over exercises.
Have a strong core and good balance
My trainer ties my stirrups to my girth with bayling twine. 😂 Good Luck Tessa!
(I also think u should try no stirrups)
The reason is often because the rider keep the knees to tight to the saddle. Try to ride without stirrups for a while and you will feel a big difference when you use them again. It’s also easier if you ask someone to lunge the horse while you ride without stirrups. The person can also give you feedback and a perspective from the ground to help you even more. Best of luck Tessa!
Keep your heels down and image that your legs are glued to the sides of your horse!