There's an old saying: "if you think you can or think you can't, you're right". But what we think consciously makes no difference in trading - it's what we believe. Beliefs are deeper in the mind (preconscious). Since beliefs create feelings (such as confidence), we get an idea of the nature of the underlying belief. Practically, it means that if you feel confident, you'll win every time because you have a positive belief. This applies no matter how good our system is. Everyone is familiar with this idea. My experience says it's 100% true.
It's easy to pretend we're confident when the belief is actually the opposite. The way to tell the difference between real and fake confidence is through feeling, as described already. What we feel is what will happen to our trade, not to the market. By feeling confident, I don't make the market go up, I make my trade a winner (regardless of what the market is doing).