Feeling sad or blue from time to time doesn’t mean that you’re depressed. It only means that your life is like the life of any other human being; consisting of cycles when you’re feeling great and cycles when you’re feeling not so great.
On the other hand, if you are depressed, you’re overwhelmed with dark feelings of helplessness, hopelessness, and worthlessness. These feelings are often accompanied by guilt and shame. You feel like a black cloud is constantly hovering over your head.