Have you ridden gaited horses before? You can't "switch modes", nor can you begin to teach her anything new until you know what she knows. Could be she is doing what is normal for her. Here's an article that might help you... MAKING CONTACT, How to use a bit, By Lee Ziegler
excerpts from the article:
"Before you can successfully develop light contact with the mouth of* a gaited horse, you need to determine what he knows about the use of a bit.* If you are faced with a horse that has been trained to go with the “traditional” 10 lbs of pressure in his mouth, at first you are not going to have much success riding him in gait if you try to ride with much less pull in his mouth than he expects.* If, on the other hand, you are riding a horse that has had only “natural” training and has no idea that any pressure on the bit can mean anything but “whoa,” you will not get very far trying to ride him with constant contact, not matter how light, on his mouth.** Ride around in an arena a little while, in a walk, feeling out what the horse knows and expects from a rider.** Try riding with slack reins, or with some contact to see which method the horse understands.* Once you have figured out what the horse expects in the way of bit use, you can begin to modify what he knows and teach him to accept light, kind, contact with the bit. "
"At first, a horse that does not understand light contact will often toss his head, throw his nose up or root “behind” the bit, objecting* to the feel of your hands through the reins.* He may also slow down or stop at the first hint of any weight heavier than the loose hanging reins in his mouth.* The* solution to this reaction is to keep your hands steady, fixed and quiet, and alternate light vibrations with your fingers on the reins when he tosses his head with squeeze/releases with your legs if he slows down, until he discovers that he can carry the bit with the weight of your fingers on the reins.* Again, it will take time for the horse to accept the idea that the bit works for more than basic stopping or turning cues."