i dont think you quite understand what drill team is. there are many maneuvers that are meant to be performed at top speed. the op never mentioned she had trouble controling her horse, just the issue of her seat. i think what she is asking is how to have a better seat at the faster gaits. op, i appologize if that is not what you are asking. as for help from me, sorry, haha but i am not much of a speed demon! maybe head to the barrel racing section as i am sure they have some pointers on how to stay seated going that fast.I would suggest slowing your horse down with your body in addition to your rein aids. It causes you to have a deeper seat, so that you should also be able to stay anchored in your saddle better as well
Meanwhile, maybe you should work on getting back to the basics in your training so that your horse listens to you better.
Good luck!