Might also be that something you are doing when riding is making her dread it?
Video would help there.
But you could have saddle too far forwards, cinching too tight, not tenting the blanket under gullet, not taking care to move mane out of way, not making sure she is not being pinched by girth, toeing her when mounting, or bumping her mouth with your hands if they aren't steady.
Could have something going on with teeth too.
Could be something wrong with your saddle or pad.
And might be you are aggravating in your voice, motions, or grooming. Might be you are babying her too much and using a "that's all right" tone, and creating this.
Horses make me a better person.