In sociology, there's a concept called the “Looking-Glass Self.” The idea of it, is that you build your self-image based on comments that you receive from other people. This ends up affecting the way that we act; if you're told all your life that you're a smart student, you'll generally perform better on exams. The opposite is true; if you're told that you are unintelligent, your exam scores will be poor. That being said, I think it's also true that the things we say to ourselves can also affect our Looking-Glass Selves. I believe it also relates directly to how we act subconsciously.
A phrase my mom used to chirp whenever my sister or I weren’t sure if we could accomplish something, was “Fake it ‘til you make it." As many of us do, I started to lose my self-confidence in middle school, and it stayed that way into high school. In order to combat this, I would majorly up-sell myself. Any time I received a negative comment or thought somebody was giving me a nasty look, I would tell myself that I was a goddess. A person who would achieve far more than anyone who was mean to her and would have more people fall for her than Miss Universe.
And it worked wonders! The overwhelming positive took over the nasty negativity and my self-esteem balanced out quite nicely. I didn't feel nearly as bad as I did before, and I came to find that I did feel more confident in myself as time went on. However, I like to think that the things we say to ourselves limit more than our self-confidence.
As I mentioned earlier, being told you're a good or bad student can affect your test scores. But I think it goes beyond that. Let me explain with another example: I've never had a good sense of time. Sometimes I couldn't tell what day it was, and oftentimes I couldn't judge the passage of time very well. When I started to work overnights three days a week, it got considerably worse and became a running joke between me and my friends. These jokes passed as most do, and I noticed that the less I thought about my inability to keep track of time, the less I forgot what day it was and how far through the week we were. Because I told myself I had no sense of time, I ended up living the lie. Just as telling yourself that you're the best despite feeling your worst, telling yourself you're bad at something can dramatically drop your performance.
It's important to point out that, to my knowledge, this isn't scientific or something you'll learn in a Psych class. It's something that I've noticed in the past few years. But, if it's worked for me thus far, I have faith that it can work for anybody else!