What you believe about yourself affects what you think is possible for yourself.
For example, an elite athlete might feel their muscles pushed almost to the point of collapse and think, “I am reaching peak performance! This is what I excel at.” I might feel that same level of exertion and think, “I’m not sure I can do this, I’m afraid I’m going to hurt myself. I should ease up.”
One person might feel butterflies before public speaking and think, “I’m so nervous, this is terrible!” while I might feel the same adrenaline rush and think, “I’m so excited! I can’t wait to do this.”