She's bracing pretty hard.. I think possibly she was ridden goofy/wrong in the past. Maybe when she was asked to move up a gait she got bopped in the mouth or the rider slammed on her back, so she's not looking forward to it.
Best way to get through to her is work on her accepting the bit at the walk, slowly pushing for a little more and backing off when she falls apart.. getting her back to accepting the bit at the walk, pushing a little more until she holds it together, then praise like crazy.
My horse did this.. but he also hopped around, leaped, bolted, let out a few little rears too when a saddle didn't fit right. Believe me when I say that I understand LOL. Just take it nice and slow. Avoid using the reins for balance; use your core.
"Strength is the ability to use a muscle without tension"