In many cases, situations do not cause all of the stress that we experience. Sometimes, our reaction to circumstances (and what we say to ourselves about them) contributes to the stress we experience.
Sometimes we are right in what we say to ourselves. Often, however, we are overly harsh and unjust with ourselves in a way that we would never be with friends or co-workers. This, along with other negative thinking, can cause intense stress and unhappiness and can severely damage your self-confidence relating to constantly feeling in a state and stress and desire for relaxation.
Negative thinking damages your performance by undermining your self-confidence. It also occupies "attentional capacity"at times when you need to focus your entire mind on what you are doing.
You are thinking negatively when you fear the future, put yourself down, criticize yourself for errors, doubt your abilities, or expect failure. Negative thinking damages confidence, harms performance and paralyzes mental skills.