The overtraining may be a result of depression, not the cause of it. People with depression often train excessively because they are going for a "runner's high" to relieve symptoms of depression. In the long run this strategy is ineffective. There are many effective treatments for depression, if that turns out to be the problem.
I will try to find more info about this, if you are interested. I remember a good explanation in a book written 3 or 4 years ago by a science writer for the New York Times. The female author's last name starts with a K. The book was about the exercize industry and peoples' obsessions with exercize, especially spinning and weight lifting. Sorry I can't remember more.