For depression to be present, it must be more than just feeling down or frustrated for a few days. Even healthy people experience depression at times, but depressive episodes last for weeks or months rather than just a few days in depressed individuals.
Some people believe that depression is a non-existent medical condition and that it is not a genuine health condition. They are mistaken it is a legitimate illness with legitimate symptoms.
A state of depression is not a sign of weakness, nor is it something that can be "snapped out of" by "pulling oneself together." In order to "snap out of it," you can't just will yourself to be happy when you are depressed.
It is impossible to overstate the importance of positive thoughts and affirmations in one's life. A wealth of information and practical steps on how to apply these patterns of thinking can be found in this book, which can be extremely beneficial on your road to recovery.