Well you quoted someone so I guess I'll quote you and tell you why you're wrong... your technique goes to sh*t if you can't control your nerves.
That said.
1. learning how to calm yourself in the heat of the moment
2. knowing how to stay calm
3. knowing how to mentally keep yourself in the game
4. it's all in your head, so read up on how to control your head.
5. meditate
Sorry Paksat, nerves and controlling yourself has nothing to do with pool, simply put, remember the things that used to wow you when you were a a little kid, and as you grow up and learn that they are not big deal anymore, same thing with pool. you have about 2000 frequent types of shots on the table, if you do not apply knowledge (i call it secrets) to each and every shot you will miss. If you want to be good player, you have to ignore all items 1-5, and learn instead,
why you miss every shot. I know You are not going to believe what i say until you reach that level, trust me i have been there done that. However, not having enough sleep, sick, or something effect your mental thinking is different subject.