Single jointed bits will collapse on the bars and have the nut cracker effect which hits them in the roof of the mouth when 2 reins are activated, and it will also have tongue pressure.
Double jointed bits use primarily tongue prssure when 2 reins are activated.
Here is the link to the Parelli Cradle Bridle and how they explain what bit for what horse, etc. If this IS a bit issue with your horse, she may need the cradle C2 or the C3. My horse will need the C3. http://www.parelli.com/product.faces?catId=42