Well a harsher bit in the wrong hands will only do more harm then good. If you do decide to go with a stronger bit, make sure its with the consent of your trainer (do you have an instructor?)..
Is your horse a standarbred or thoroughbred..? (I can't really imagine a OTTB having an extremely fast trot, though I'm sure some do.. They like to take off when you canter!)
Collecting the trot should slow her/him down. You can do this by half halting on one rein then the other, until the horse reaches the speed you desire. You can also do one big half halt on both reins, but it generally isn't as pretty.. and sometimes horses get mixed signals and start to walk. Make sure when you half halt, you lightly squeeze with the insides of your calves to tell your horse to keep moving. Combined with the message from your legs, the half halts will tell her to slow down and not stop.
You can also do a lot of circle work, figure eights, and serpentines. Aside from the added benefit of wrapping her around your leg.. Horses generally slow down when they bend. I would start with a large circle and notice the difference in speed, and progressively work toward a smaller circle until she is trotting at the speed YOU want. Once you find the right sized circle, keep on that circle for a long time. Then change direction and circle the same size in the other direction. When you feel she gets the message and isn't trying to rush, take her off the circle and around the whole ring. If you feel her start to rush again, circle her. Repeat everytime she starts to rush.
Horses generally rush more on the long sides, so you should be prepared to half half down the long sides.
Things you could be doing:
-if you are tense or nervous you might be gripping very tightly with your lower calf. If your horse is sensitive at her sides, she'll take that as a message to move fast! You should always have contact between your leg and your horse, but it should be a relaxed contact. Relax your leg, and make sure your message is clear when you want her to move on or slow down.
-Make sure when you ask for a transition up (walk to trot, trot to canter, walk to canter) you always use your voice first, legs second, and artificial aid only if the first two don't work. If she is sensitive, she may be worried that you are going to use your whip and feels the need to rush.
-Take your spurs off if you ride with them on. You might not realize your using them when you don't mean to. (If that's the case, on the flat you should make sure your toe is pointed forward!)
-Make sure you aren't stiff, if you're stiff it can affect the horse in different ways. If you are stiff through your back-shoulders-arms, the horse will generally seem slower and often break its gait. If you are stiff through your calves-thighs-hip (or even thighs-hip-back).. you can't effectively use your seat, which not only aids in driving forward, but also transitioning down. Sitting deeply and "pushing" your back-hip into the saddle will help in your transitions down.. Similarly a mild form of that should help slow her down at the trot.
-She may be nervous, regardless of how you ride.. Some horses are just constantly on edge. Give her something to think about by doing a lot of pattern work, circles, figure eights, serpentines. Do a lot of transitions. If you think she's the nervous type, talking to her and praising her (when she does well) will help put her at ease.