"Is there anything else I could do to thicken my face and soften my soul?"
Yes - decide that you won't take anything the client says or does as a personal attack.. works wonders.
Figure out what they want and give it to them, and keep looking for other clients to work with..lather, rinse, repeat.
Recruiters have to master this "inner game', or this line of work will kill them.
Aside from that, pick up a physical activity like running, weightlifting, etc.. that will help.
As for me, I like martial arts..hitting things is good for the soul, and getting whacked in the head with a stick develops humility and sharpens the focus at the same time. Seriously though, you need to be involved in something that teaches you to control the emotions and thoughts.
Good for you. There is nothing like ccing others when responding to written fussing. It's called "murder by memo", very effective, much cleaner and sends a loud but subtle message that you are taking the high road. I have several times had the joy of watching people who do that sort of stuff be sent down the road. It never happens as fast as we wish it would, the mills of God grind slowly but they grind exceedingly fine.
The only behavior we can control is our own and when we do the freedom is glorious.