I'm confused

Having researched the symptoms and considered my feelings, according to much information on the web I believe my depression is mild (although it certainly doesn't feel like that sometimes!)
However, all I have read seems to suggest that mild depression only lasts a few weeks or up to 6 months with no treatment. But I have felt this way for 3 years!
Is this because my depression is deeper than mild purely because of the long time it has been with me? Or is it because my depression IS mild but I can't shake it off?
I do realise that I seem to come out of my depression quite regularly and start to relax. let my guard down and dare I say it - start to feel a lttle bit happy. But is seems that (due to many previous upsets) I just know that it's all going to come crashing down. And it does. Sometimes within days or hours. So I almost resist being happy.
Does anyone out there feel the same?