I can't give you much advice when it comes to spooking, but I do know a lot about fast horses.
You want to get your pony behaving from the moment you get on her back. Speak to her softly and gently tug the reins while warming up at the walk until she is paying attention to you. Watch her ears. If she is listening to you, in the least, she should have one or both ears flicked back.
When you ask her to trot, use a very light aid. Simply flex your calf muscles or squeeze gently until she breaks into a trot. Keep yourself calm and relaxed; she can sense your emotions. Check to make sure she's still paying attention to you. Also, make sure you practice circles, serpentines, and figure 8's. Bending will help your pony listen to you and focus on turns instead of simply rushing around to outside of the arena.
When controlling the pace at a trot, it isn't only about half-halts. It's about your seat and your posting. If you slow down the speed of your posting, she will slow slightly to match it. Keep your shoulders tall and square and sit back. If you feel her getting out of hand, say 'whoooaaa' quietly and give a small half-halt. And again, do not just rush around the outside of the arena. Do lots and lots of circles, big and small, and control them. One excercise I find very effective is to ride in a circle at one end the arena, spiralling smaller and smaller each time and then spiralling back out.
Do not canter until you can keep a controlled trot. If your pony is rushing at a trot, she will rush at a canter. Wait until you have reached the desired pace. Once you are ready to prepare to canter, slow your pony to a walk. Encourage her to stretch her neck and relax for a minute or so. Then, pick up a nice easy trot. If it's not what you want, don't let her canter. Wait until the pace is relaxed and even. The transition should not just be a run into a canter.
At a canter, sit full seat. Half-halt in the corners. If she starts to get out of hand, pull her into a tight circle and give a firm but quick half-halt in the next corner. Resist the urge to lean into circles, and make sure your inside shoulder does not collapse. Circles, circles, cirlcles and tons of bending. Use your inside leg to encourage her to bend around turns.
When all else fails, if she completely takes off with you and refuses to stop, drive her into a small circle and then halt her directly infront of a wall. Never let her get away with it.
Good luck. I hope I was helpful.