I had the same problem as well! When I talked to my instructor I went right back to the basics, I went tackless! Not only does it help to soften your hands and not be so hard your horse but it teaches you balance and not to rely on the reins.
I saw some exercises to keep the hand steady, It might help you! They were: Having one ball in each hand (while riding on the lounge) and trying to hold the balls doing sitting trot and canter, them you start to move your hands in diferent paces, one goes down the other goes up, basically moving independently!
You can do it without the ball, but it really helps to keep the hand steady and moving independently from esch other and from the rest of the body
Hope it helps
You can use a collar to force yourself to put your hands down and don't pull on the reins. You can also try to ride without the reins and force yourself to slow down your horse with your back and not your hands.
Do you have any idea why you do that so often? Are you scared of something?
Good luck :)