Why do you get "off days/bad days" of golf?

One day, i will be striking solid shots with my driver, 3 wood, irons, wedges, and even the putter; and therefore playing my best ever golf. However, then the next day or days after, i would usually hit inconsistent shots with all my clubs, causing I to have a short fuse or get easily tempered. Consequently, what can i personally do, to be able to be more consistent, prevent any off days/ hit more solid, and not to be easily stressed..Thanks, as i have a tournament this saturday!!!