Horse gets angry when you put your left leg on?

My horse has a problem of falling through her left shoulder. But whenever you put your left leg on she throws her head up, pins her ears, and swishes her tail. This happens a lot more in the trot and canter though but still happens in the walk. If you get her moving without falling through her left shoulder she is wonderful but that only happens for a matter of seconds. Also if you get her left shoulder moving straight and ask for a faster gait like the trot she can't pick it up for her life without falling through the shoulder. How can I fix this? Is she in pain or just mad?