*busting out her DSM-IV*
AOXLiz: Yes, you are describing clinical depression. And it was very brave of you to describe what you are going through. Society has a tendency to impose the idea on us (especially women) that we must be happy all the time. And yes, sometimes we feel like "ugh, what's wrong with me?" when we aren't happy. However, when your symptoms are lasting two weeks, most of the day, nearly every day, that becomes a problems.
Although no one on GC knows you irl, it is evident by your posts that you are a wonderful, kind person and you do not deserve to suffer!!!
A clinical psychologist who is a co-investigator where I work studies depression. He said that he (along with many other prominant people in the field) are finding more and more that rumination is a sign of depression. When you said that you have "constant nagging in your head that you cannot get rid of" that reminded me of him and his work. Several key points are:
-People who are depressed think about depressing/unpleasant things much more and much longer than someone who isn't depressed.
-Your brain and it's mechanisms can be likened to a muscle. When you use a muscle more, it grows stronger, works harder, etc.... The more you think about depressing things, the "better" you get at it and the more often you do it. Your brain's chemicals essentially create more effective pathways for negative thinking.
So, what does he suggest? Practicing "mindfullness." Although I do not know much about it, what I gather is that it is choosing to do a task and then completely concentrating on it. A list always helps to fill up your day with tasks to concentrate on. As Dionysus said, exercise is a good thing because it gets the endorphines flowing.
Good luck with everything. Although I may not know you, I care and sympathize deeply with what you are going through.
Although this post could have been PM'd, I thought that maybe what I have learned would help someone else.