Make sure you are sitting back and square in the saddle. My trainer did and exercise with me once for the same reason. What we did is we halted and dropped our reins. We placed both hands under our bottom. If we leaned forward or back you would fell the weight come out of the saddle. But, if you are sitting up straight you can feel all the weight in the saddle. Also act as if you were a rock sinking in the oceans ;make yourself heavy
Sit deep, heels down, plan your hands below your belly button (shorten your reins first), and halt. As soon as you feel the nonsense begin, halt and back up a few steps.