There's a good chance the reason she won't bend or go on the bit is not her being stubborn but her being unflexible and not using her body properly. Do lots of Figure 8's, changes of directions, circles, spirals and serpentines. Make sure you are riding her equal on both sides. Go over poles and raised cavaletti.
Hope this helped,
Figure 8's, flexing to make sure she isn't stiff. When she rushes bring her into a circle large or small keep her going until she relaxes. This is what I have done with my sisters Mare while my sister was away and she turned out fantastic!