Are you having trouble with both the posting trot and the sitting trot? You may be too tense and trying too hard. If you relax, your body will be better able to balance itself. The other concept that is not taught enough is to feel what the horse under you is doing and match your rhythm to his. Eventually, you can learn to control his speed and trot by changing your rhythm and he can learn to follow your rhythm instead of you following his rhythm, but it has start with the rider sensing and matching the horses rhythm. If the rider’s and horse’s rhythms match it is more comfortable for both, and once the horse figures this out, he can learn to match the rider’s rhythm.