My mare has been constantly shaking her head and sneezing which then pulls me out of the tack during EVERY ride. I know she isn’t testing me and it isn’t the dust or sun. Any ideas

could be allergies, try getting a little screen to hang from her nose band that covers her nostrils
I made is allergies or an irritation in the nostrils or nose.
It could be physical, maybe check with your vet?
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