Any mental illness is tricky to diagnose and treat because doctors and therapists have to rely solely on patient reports of their symptoms. There is no "test" per se other than verbal questionares. I've also heard depression in particular referred to as "The common cold" of mental illnesses, I would presume because so many people will go through at least one bout of Major Depression in their lives. It gets even fuzzier when you start considering how many mental illnesses/personality disorders have "depression" listed as a symptom. Cutting to the chase, I think you should consider seeing an MD/DO, there are many medical interventions out there that can ease symptoms of depression and I'm not just talking about medications. I remember watching a documentary about psychology a few months ago that mentioned passing really powerful magnets over people's heads to treat depression. The brain is an organ and it can malfunction just like any other organ.