You need to ride as much as humanly possible, on as many different kinds of horses as possible. It takes an insane level of fitness to be able to jump at that level, and that only comes from literal hours per day in the saddle. Don't be snobby about the horse you ride. Even schooling the grumpy 13hh lesson pony can teach you something if you are open to any opportunity to learn.
A good way to do this is to work under an excellent trainer, or do working student positions. Expose yourself to as much knowledge as possible; go watch local shows and walk the courses if they let you, join the ring crew at some local shows, audit clinics, etc. Don't be afraid to put in the less fun work to get where you're going. Most people will exchange their time and hard work for extra riding and training.