A setback is anything you wanted to happen that didn't happen or anything you didn't want to happen that happened. That means you experience setbacks several times a day. It is a rare day that everything goes exactly the way you want.
When a setback occurs, you explain it to yourself. That is a natural and unintentional result of any setback. You explain what caused it. You don't explain events deliberately; usually you attribute the causes of events automatically. The automatic, natural, habitual way you explain events is called your explanatory style.
For example, if Jim and Sue lost their jobs from the same company on the same day, they may have two entirely different explanations for why they were laid off, even if the company gave them exactly the same reason. Jim might think, "The economy is bad. That's why they laid me off." Sue might think, "They didn't lay off everyone. They must have chosen me because they noticed I didn't try very hard." Same circumstance, different explanation. And the explanation each person made was according to their explanatory style.
Look at those explanations again. Which one is better? Which explanation will dishearten the person and which will not? The answer will not be obvious to most people.
The consequences of Jim's and Sue's explanations are significantly different. Jim may feel defeated by his explanation and lack motivation to try to find another job. His explanation says the cause of his setback is widespread and out of his control. Sue's explanation, however, may cause her to decide to get a job she really wants and to put her heart into it. Her explanation was more specific and the cause is under her control.
Explanatory style will determine whether a person is demoralized by a setback or unaffected by it or even motivated by it. And this difference, as it accumulates momentum with the daily setbacks we all experience, greatly alters the trajectory of a person's life. It will also have an impact on health. Read more about it in Martin Seligman's Learned Optimism, David Burns' Feeling Good, and Adam Khan's Antivirus For Your Mind.
The approach is scientific and effective. It isn't positive thinking. There is no need (or desire) to try to believe something you don't actually believe. The approach is to think more accurately. The aim is to reduce the number of mistakes we all naturally make in our thinking, and thereby reduce the amount of helplessness we feel. When we stop making ourselves feel so helpless and hopeless, our natural determination and fighting spirit begin to rise, and success in the long run becomes more probable.