What a person things about himself or herself has the potential to make it come true. If a person thinks he or she can't accomplish something, then that person may give up. If a person does not have self-esteem, the person may become insecure. If a person is quite ill, but believes he or she can overcome the illness, the belief may indeed heal the person.

Is this a proverb? It could mean: A person may have an idea about himself that will prevent him from doing good work (because he is not paying attention to the work he needs to do). In other words, you cannot do good work if you are busy worrying about yourself.

There is something known as "fear of failure". It involves someone being so afraid that they will fail at something that they don't even try. This may be what your sentence is referring to.