Over jumping is when you move ahead of the horse instead of with him, thus ending up that you would be crawling up his neck. When someone over jumps, their seat would be suspended over or infront of the pommel of the saddle. Another sign of over jumping is that your leg has slipped back, which often happens when a rider throws the rest of his or her weight forwards and therefore moves the leg back to stay balanced.
Try having someone take a picture or video of you jumping from a side angle. See where your seat is and your hands are. Your seat should be poised over the center of the saddle and ultimately should still be over your heels. Your hands should not look like they are reaching for your horse's ears.
Hope I was helpful.