We see what we want to see. Sometimes we want to see what we'd prefer not to see. Sometimes what we see, is something we need to see.
But it is true?
Perception creates reality. If you repeat something often enough, people will come to believe it.
Have you ever been somewhere and the people you're with start a rumor about a particular person? Whatever the rumor is, suddenly becomes the truth. It doesn't matter if it started as a lie, everyone will treat that person according to that rumor. If the woman has been labeled as a "bitch", people will generally treat her as a bitch. And then, in turn, she will begin acting like a bitch.
Every morning, tell yourself that you're fat, and in time, you'll feel fat, then create situations where you gain weight. If you tell yourself that you're hurt, you will develop pain, or find some way to be hurt.
Now, how can we use this to our benefit?
Tell yourself that you're happy, that you're good, that you can achieve, that you can be a better person... and they will come true. As you prepare for an exam, tell yourself that you will pass, and really believe it. You should find that you retained more of that information. You should feel better about that exam. You should feel more confident about how you'll do. You will do better on that test.
Many times, we are where we are, because we put ourselves there. We don't realize that there are forces that we can control. We believe we are in the grip of fate, and cannot change what is happening to us. But we can.
There's a saying, "Our deepest fear is not that we are inadequate. Our deepest fear is that we are powerful beyond measure." We create the trappings that hold us down. Sometimes, we come to love and depend on those trappings. But we must learn that we can progress, and go beyond them.