It isn't easy to make those transitions. I think if you really think about centering yourself around your core muscles in your abdomin, this will help give you some more stability and helps to cue the horse to come back to you. It's especially important in the down transitions.
I think in your post you are talking more about your own seat trouble, right? Gripping with your legs can make it harder for you, so really keep those abs engaged.
And , of course, the better your horse is prepared to make the transition, the smoother he will do it. One thing that I use when transitioning up or down is to think about breathing out on the transition.
Those are just two small ideas that might help. But just to let you know, I think many riders have trouble makeing those transitions, so don't feel bad.