Try to think of your shoulders and arms as two different parts. You want your shoulders to be still and not pumping but you want your arms to be elastic. Also I agree that putting your hands on your horses neck to feel the movement first is very helpful!
Try putting your hands on their next for a bit to really feel the movement while riding. Then when you back to your normal hand positioning you should try to follow their head movement, don't overthink and try to move before your horse does. And a huge tip is relax your shoulders! When your shoulders are relaxed your arms are free to follow the horses head