Frustrated: Feeling or expressing distress and annoyance, especially because of inability to change or achieve something, being prevented from progressing, succeeding, or being fulfilled.
If you are frustrated because your horse messed up on the 21st pivot, after 20 perfect pivots in a few min span. Your horse is frustrated that he gave you the 20 perfect pivots you asked for and his only reward was for you to ask for another. (Which now with his fumble has probably turned into a few dozen more).
If you are frustrated with your horse bucking at the canter. Your horse may be frustrated that every time he picks up the canter your saddle digs unbearably into his shoulders and back and the only way he can get you to stop cantering is to start bucking.
If you are frustrated your horse won't do (insert just about anything here). Your horse is frustrated because he doesn't understand what you want,that you keep asking exactly the same way, and that when he does try to try you will not acknowledge his slightest effort in the right direction.
At the end of the day if you are frustrated, your horse is too.
And if you find yourself frustrated the best thing you can do is go to a trainer, clinic, very accomplished friend, and ask for a fresh set of eyes on the situation.