Confidence, they say, is everything.
Who are "they"? Well, internet meme creators, of course. Seriously though, there's merit in such quotable goodness, especially in the insanely-pressurised world of pro wrestling. If a worker doesn't believe in themselves and stride through the curtain with determination and their head held high, then their performance will definitely suffer.
Wrestling is a tough business that needs some semblance of courage and pretence even when one feels low inside. It's performance art, and no wrestler wants to hear cruel chants like, "You F*cked Up" when they take their eye off the ball and botch or raise the ire of company figureheads backstage for failing to meet pre-match expectations.
Often, people can be tougher on themselves than others, and that's another battle entirely. It's really not easy to manage self-esteem, pressure and anticipation 200-300+ nights per year. That's a wrestler's lot; they don't have time for confidence wobbles when they're on the road so often.
That's probably why so many of them have suffered through a crippling crisis of confidence or had to wage personal wars on debilitating mental self-doubt...