The first thing you should do when you have serious symptoms such as episodic depression is get a full physical exam. There are simple medical problems that will produce the symptoms you related.
You didn't say what you do on the days before you experience depression. That can have an effect. There is a simple phenomenon that will cause depressive symptoms. It was discovered when it caused mental events for knowledge workers in the 1960's. They had brief periods of confusion and psychotic-like behavior. Fear, paranoia, panic attacks, depression, and thoughts of suicide are some outcomes of this phenomenon.
While you are getting that physical check up visit the panic attacks page at VisionAndPsychosis.Net. This will not cost anything and does not involve treatment of any kind.
If you do find activities you have to cause exposure slight changes will eliminate it. Incorrectly located home computers are a likely source of this exposure. Working in small business offices with out Cubicle Level Protection is another source.
Recent onset symptoms should stop within a week or so. But long term exposure will not be easily resolved. You have to unlearn the depressive behaviors.
http://VisionAndPsychosis.Net
Of course depression is not a constant state, just like other illness, it has different levels of severity, different causes for people and manifests differently. Dystimia is a form of depression that a lot of people have that goes unrecognized but still causes problems. Look into it because it sounds like something you experience. Its a form of mood disorder also. Depression and the like can flare up for a variety of reasons, like stress or life changes. Don't forget that our bodies go through significant changes about every 7 years, both physically and mentally and it can be distressing. Please educate yourself about what is happening. It sounds like a combination of stress, dystimia, general aging. All of this is being influenced by your environment and how you cope with various things. You can always add extra vitamins to your diet that truly help with feeling better. I am a huge believer in the vitamin B family including niacin, B6, B12. You would be amazed at how good you feel when you supplement them in your diet. Let me know your thoughts.