Words mean things. Words affect us.
While we never notice the process, our brains filter every word that we speak or hear, and adjusts our mind to comply with what it’s being told. If your brain hears “you dummy!” often enough, it assumes that it’s true, so it lowers your internal expectations to that level. And when you mess up, your mind scolds you for even thinking that you could get it right in the first place. You are a dummy, remember? And dummies always mess things up.
If you are one of those who mock that concept by saying something cute like “I’m gonna think myself seven feet tall and good looking,” then carry on. I won’t waste time trying to convince you.
But what we hear does influence us — especially if those words are spoken by someone important to us.