My horse is developing a habit of reaching his head to the ground when we’re working on the flat– he’s looking for his weeds that he’s obsessed with… It’s incredibly annoying. What I’ve been doing with him the past couple of rides, and it seems to be working well, is when he puts his head down, I apply a firm, not harsh, but firm pressure on both reins, and when he brings his head up, even just a little, I release him immediately. It teaches him that hey, if you put your head down there, it’s not going to be comfortable, but if you bring it up, you’re going to get a nice big release that will make you feel good, and it will be a lot more comfortable. If he’s only doing it while jumping, then try approaching the jumps at a trot and doing this.
Also, is all his tack fitting okay? Make sure to check that. He might be dropping his head and speeding up in an attempt to alleviate the pressure and pain, and only does it when jumping because that’s when it hurts the worst? Something to consider.