I have a severe depressive illness. I've been fighting it since about age 6. I don't know how much of it is brain chemistry and how much comes from an extremely abusive childhood. A case can be made either way, but one of the more compelling facts pointing to brain chemistry is that a cousin of mine has it too, and she was decidedly not abused.
Our mothers are sisters. I was 14 when this cousin was born, and my aunt had most certainly learned from the mistakes she saw my mother making with my siblings and me. There was none of that in her house. My cousin wasn't constantly uprooted and moved from school to school just because her parents couldn't afford to pay the rent. In fact, her father saw to it that their income and household remained stable. Her parents didn't get divorced, and her mother didn't remarry a whole string of abusive men, one after the other. Nobody got drunk. Nobody swore at her, beat her, or called her stupid. Nobody molested her.
Yet she has the same illness I have, and takes the same medication I take. When she was first diagnosed, it baffled her. "How can *I* have mental illness like you have? I had a great childhood and great parents. Those things that happened to you didn't happen to me!"
But mental illness doesn't care.
I said all that to say, depression is not a sin. Many people confuse it with sadness or ingratitude, but that's not the case at all. When my brain isn't working right (and today it isn't) I can still love the Lord. I can still be glad God sent His Son to die for my sin. I can still count my blessings and acknowledge what He has given me. It's going to be more difficult, kind of like running through a pool filled ankle-deep with molasses, but it can be done. God knows I hurt, and He understands when I don't bubble over with enthusiasm the way other Christians may do.