Dealing with nerves is unfortunately just part of riding. Every single rider out there, even Olympians, feel nervous at times. Whether it's a cold day and your horse feels on the verge of blowing up, or you're cantering down to a fence worrying about seeing a distance, most riders will react by holding their breath or taking small shallow breaths. The problem with that is your body then stiffens, signaling to your horse that there's something to be worried about. Also the brain needs oxygen to function correctly. When you hold your breath, you are letting your tension get the better of you and your thinking will be clouded and your ability to sense distances, for example, will suffer. Next time you feel freak out mode coming on, try taking some deep breaths using the following method, then check in with yourself at different points of your ride to make sure you're still taking nice, full belly breaths.
Inhale deeply. Your abdomen and stomach should expand as you inhale.
Hold your breath for 3 to 4 seconds and think to yourself, "I am melting into the saddle" or "my body is relaxed."
Exhale and think, "I am calm." Imagine all of the tension leaving your body and mind with each exhalation.
Breathe normally for 10 seconds.
Repeat steps 1 to 4 three or four times.