Although this mare was started under saddle about 5 years ago, it sounds like she is still very green and has not been given a good foundation. You don't mention your own riding experience, but because you are asking for help with this situation, I suspect you are also "green". The only way your horse can communicate is through her behaviour. She is not being "naughty". She is showing that she is stressed and not enjoying being ridden.
There can be several causes for your mare's behaviour. It could be because of a physical pain or discomfort being caused by ill-fitting tack, her teeth needing to be floated, muscle or joint soreness, or a chiropractic issue. Some of these physical problems are not apparent in a routine vet exam. A massage therapist or chiropractor can identify problem areas, if there are any. Another cause of her behaviour can be how
she is being ridden - any tension, stiffness, imbalance or crookedness in the rider negatively affects the horse. The more sensitive the horse is, the more she will be affected by even a small amount of these problems in the rider.
Has anyone else ridden your mare since you got her? If not, ask a more experienced rider or a trainer to ride her and see how she goes. Get help from an experienced trainer who can help determine the root cause of your mare's behaviour and then work with both you and your mare to develop a better partnership.
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