Hmm..well don't worry about your position. Usually when you worry about it, that's when you mess up, so my advice is to just imagine your seat as "jello" and just moving with your horse like a wave almost. Just relax, and enjoy yourself! Just put your weight in your heels, because that is the key to good balance.
Speed wise, I had the same problem. One of my trainers suggested half halting first, and doing little squeezes. If he doesn't respond, being him right to a halt, and back him up. That'll gain you some respect, and he'll spend more time "thinking" about when you're gonna say "whoa" instead of ignoring you.
"In the end, they're only going to remember you for the times you smiled when you had every reason to cry."