What bit did you change her from? If she needs flexion at all, beit left and right, and lateral, I'd suggest using a ported correctional. Sounds to me like she was running through the snaffle, and to be honest, I wouldn't be trying to learn with a horse and teach it correct cues in a snaffle.
One good exercise you can do with her is walk her in a small, tight circle. 8 ft diameter or less. Use pressure on your inside foot to bend her ribs around your inside leg. You will need to slightly tip her nose INTO the circle, and make sure she has at least ONE ear AND ONE eye on you (just the corner of the eye... you don't want her head cranked around too far). Once she has the ear and eye on you and doesn't even make the slightest glance to the outside of the circle, lift up on the reins to shift her weight to the hind end, and apply pressure with your outside leg while relieving pressure on the inside, to encourage her to turn around and reverse the circle. Do the reverse, then the same exercise on the other side/other direction.
This will teach softness.
Soon, you will be able to expand your circle to 20' diameter at a trot, nose tipped in, ear and eye on you, ask for a whoa, lightly direct the nose around, relieve pressure on the inside leg as you apply to the outside leg, and encourage the turn.
Once she gets this down to an art, you will notice how much softer she is, and how much more accepting of direction she is. For this exercise, you will probably need a bigger bit than a snaffle to reduce having to pull on her face or rip the corners of her mouth if she refuses.
I suggest something like this: http://images.auctionants.com/5348.jpg
Make sure that you get something with swivel shanks to encourage left and right. A solid port bit will not give you this.