First of all, I want you to know that there is nothing to be afraid of.
There are things that happen in life that pull us down from inside. Things that at their moment we wouldn't be able to think they can do such a thing.
I'll give you a personal example, of a close friend I had who would try to "educate" me in a scolding manner. At those moments I wouldn't feel the scolding... until time would pass. He has with time devastated me, and I'm fighting. It's not easy, but I am fighting.
Depression, like other mental illnesses, are a result of us vs the outside world.
There may have been things that happened, even the smallest ones, that together create a strange, gradual feeling of pulling down. Because it can be many small things doing it, you have no idea what is going on.
I know what you're going through is not easy on you, but you're not alone. And again, there's nothing to be afraid of.

Do you see a counselor or a therapist? With time they can, for instance, help you find the things and hidden feelings that put you down, and allow you to understand the feelings and why they make you feel the way you do.