I agree OP, it's a terrible feeling, especially if you feel like you sunk to a low.
I am working on learning to post and change from western discipline to English. I know that this is new to both of us, and I am causing much of the issues with balance, but this one night Sam was a dork, mouthing his bit, trotting sideways, just being a butt.
I felt myself losing my patience over his behavior and then bang! I gave him a solid pop with the reins. He had no idea what to make of it and I felt terrible for taking my frustration out on him.
I went around the ring two more times and ended our session on as positive a note I could.
The next riding session, he acted like nothing happened. I, however, managed to keep my patience in better check. I still felt like heel.