I had a big TB mare that did similar things at that age – spooking, bolting and bucking so huge I would get launched through the air. It was terrifying! I found that when I immediately turned her in a small circle using one rein during a bolt or buck instead of pulling on both reins she gave up quickly. After only a few times of turning her in a tight circle she gave up on the spooking, bolting and bucking cycle. I currently have a WB mare who was very spooky as a 5-6 year old. It is still a work in progress, but I am finding that if I am not the leader of our pack, she will be and she will decide to spook. To be a good leader I have to ride in black and white – no wishy washy signals or responses. I have to be firm and fair. I also have to stay relaxed and not grip with my legs, tighten my seat, or have tension in my arms and shoulders. She feels the tension immediately and spooks. Also, I find that if I lean back behind the motion when we are going past a monster, she either doesn’t spook or it is mild. I think leaning back puts my seat in a driving position and also makes me sit very deep. If I tip forward it is just asking her to spook! Lastly, I am finding that as she is getting better trained she is not blowing through my aids to spook as much. She was the worst as a 5-6 year old when she was challenging my authority and trying different ways to change the subject and get out of work. She is 9 now and is really fun, so don’t give up on your horse. They do grow up eventually!
PS – I agree with Lynn A regarding turnout, feed, and a good trainer. All will help with the issue.