I always start my young horses in a simple metal loose ring snaffle. How old is she? How much training has she had? If you prefer something that is a little softer on their mouth, you might look into a french link or a billy allen snaffle. They help prevent that "nutcracker" action. Also, how in the world is she chewing the rubber off? Is the bit placed properly in her mouth? It should rest between her teeth (in front of the molars and behind the incisors). I would work with her to get her more supple. Stay in a small arena or roundpen away from other horses for schooling and work on her mouth. I like to start trotting medium sized circles and every 3/4 of a round, I will ask for a stop. I ask very softly with minimal contact with the bars and keep steady pressure until they stop. The instant that their feet stop moving, release the pressure on their mouth and give 'em a scratch. Soon, they are aware of even the slightest movement of your hands and a feather touch will stop them in their tracks. If this doesn't work, try teaching her a one-rein stop and work on her bending and giving to the bit. Hope this helps.