Do you also ride when you are not taking a lesson? Maybe just getting out on a trail or even a nice relaxed ride around a pasture would help. Breathe, drop your shoulders, loosen your arms, let your legs relax down the horse's sides, sit up.
To me it sounds like you are tense and/or maybe hanging on to the horses mouth for balance. When this happens to me I have to take a deep breath and loosen myself up and then go on.
If you ever find yourself in a fair fight, it's because your tactics suck. ~ Marine 1SGT J. Reifinger