Calming methods on horseback?

[HorseFreak]

I expect a lot out of my horse. It's hard for me to teach him new things because when he doesn't do it correctly, I get frustrated and reprimand him for the littlest things. Usually after a little I just get fully frustrated with him and it shows in my riding. Lately i've just been stopping and letting him rest while scratching my arm and making myself bleed, and after i'm noticeably a little calmer. I was wondering if anyone had any good calming tips so I don't have to scratch myself? The deep breathing doesn't work for me, especially since I have minor chest pains so it hurts to breathe in too much.
@Silver Wasp, he is an old 21 year old horse I got when I had been riding for a month(long story). I had and have an instructor but I didn't teach him some things, so now I'm trying to teach him it. He is not green.