If it is just a behavioral thing maybe you can try this. When she drops her head and starts to gallop off, sit back as much as you can, lift your hands straight up so the bit isn't pulling back in her mouth but up, she should stop. Then back her up, make her stand calmly, and start again, walk, trot, canter, and if she takes off, do it again. She should learn quick that changing from canter to gallop means a very uncomfortable stop and backing up so cantering nicely until asked is a better option.
Then also make sure you aren't boring her to death with podgy little canter circles, round and round in an arena. Maybe take her out to a field and let her loose a little and do a controlled gallop with her occasionally, if you aren't already :) It can be alot of fun for the both of you to just relax and not worry about the show ring.