You haven't given us much info to work on, but I'm going to make some educated guesses and give some general responses about your stiffness:
First, evaluate your position: make sure it's functional and relaxed, that you're not forcing your heel down or your shoulder back to conform to what you think your position should be. Anytime your ankle, knee, hip, shoulder or elbow stops opening and closing and acting like a shock absorber, you'll become stiff and braced - non functional. Position is designed to put the joints of your body in the ideal configuration to follow and influence the horse's motion. As soon as that stops happening, your position is no longer functional, no matter how textbook pretty.
Next, make sure you understand how the horse's body moves at all gaits (the sequence of foot falls, and how the head and neck, back and barrel move in response to the foot falls), and that you have a true following seat and following hand and arm. If this concepts are new to you or you're not sure, find an instructor who can work with you just on this alone for a while.
Finally, breathing. Even riders who have good functional position and a following seat and hand get stiff at times because they hold their breath or breath shallowly. Sounds stupid, but try this on your horse: Get a good, long, relaxed, end of the ride going back to the barn walk. Notice how your seat and arm is following. Hold your breath, and notice what happens to the following.
Even if you force yourself to continue the following motion, it will be stiff and jerky as long as you're holding your breath. So concentrate on deep, regular controlled breathing anytime you feel like you're stiff. This also works when you feel your horse tense - push out a big exhalation, and then concentrate on regular breathing, and it sends the message to the horse that there's nothing to worry about.
If you can give me more info about specific problems you're encountering, I may be able to give you more targeted suggestions.