For me it's all about empowerment. If I know what to do in certain situations, I'm able to manage bad situations instead of react to them. An example, if I know I can stop a bolting horse 99% of the time, (nobody's perfect) I'm not going to be overly concerned about it happening. Since horses are creatures of self fulfilling prophecy, I've therefore reduced the chance of it happening.
When you're cantering your horse and you start worrying about her spooking or running off, whether you realize it or not, your body is giving her cues to do just that. With the help of your trainer, try to tune into everything your body is doing. You may find that you take a little stronger hold on the reins or lean a little forward or back, or cling with your legs when you begin to anticipate. If you can keep that from happening, it will go a long way toward preventing unwanted movements by your horse and will also help your confidence.
You just have to see your distance...you don't have to like it.