Legnthen or shorten your stirrups. They could be at a bad legnth causing you to get off ballence
Two points t I. Walk and trot with out stirrups
Work on your core. if your core is tight it makes it easier to balance.
Also your balance is in your stirrups
Practice makes perfect but remember the movement is in the hips. It's that delicate balance between being ahead and behind the horses movement. Don't throw yourself away if the horse takes off too soon or not soon enough they will find their perfect strides. Don't know if that helps but that is what has helped me :)