Depression is more that feeling blue (i.e. down in the dumps). It is also a matter of exhaustion and fatigue. It is not only psychological but physiological. You don't feel energetic and you may feel slowed down. It is not a matter of being deprived of calories because depression "depresses" the whole body. You feel fatigued all the time and its difficult to motivate yourself. Thinking can be tiring and making a decision may seem impossible. Concentrating and remembering things may be beyond your capabilities. You may feel restless, anxious or irritable and unable to focus. You may not be able to accomplish your work properly and could be ineffective in other areas. A mental burden weighs you down and it seems you must carry it wherever you go. Activities you previously enjoyed no longer interest you. Its like someone's turned on a low frequency wave generator which neutralizes all positive and meaningful expression and leaves the world with only superficial activity. You feel empty inside. Another symptom of depression is not sleeping very well or sleeping too much. God's promise is "he grants sleep to those he loves" (Psalm 127:2).
At some point you must exercise your will power to give yourself a push to get going. But shelving this phase may reinforce not trying, and this attitude may become self-perpetuating. A rationale for it may be necessary. If you can apply your thought processes to solving a real-world problem then practical feedback will encourage you. If the project has a realistic application in your life then it will build you up because you perceive that you can succeed at something. It provides a direction to go in instead of around in circles without any goal. But without pacing yourself you'll "jump out of the frying pan and into the fire." You need periods of rest and exercise. Pushing yourself too hard unnecessarily for too long causes fatigue and loss of ground gained. You must have some realistic long term goals, but locking yourself into them prevents the freedom and flexibility you need to work yourself out of your depression. "Man shall not live on bread alone, but on every word that proceeds out of the mouth of God" (Matthew 4:4).