Depressionfag here.
The way depression manifests itself is slightly different for different people, but in general, depression is caused by a chemical imbalance in the brain. Saying "just get over it", or "try being happier" is about as effective a treatment as saying "walk it off" to someone with a broken leg.
Personally, my depression manifests itself physically and emotionally.
Physically, I am always tired. Being in frustrating or difficult situations especially exhausts me. I'm exhausted to the point where I constantly oversleep, and feel like napping throughout the day. Cognitively, it's like if someone were to go a day without sleeping, I experience the same mental fog and exhaustion constantly.
Emotionally, it deadens me. It's not specifically being "sad" about everything, it's more being "numb". Things that should make me happy feel meh. Things that should upset me feel meh. Everything feels exactly the same to me.
This tends to mess with my daily life a lot. I don't feel the urge to get out of bed, and would rather just sleep all day, even if I know I have something to do. I skip class, work, and assignments, even though I know they're important.
This juxtaposition is the worst part, and it's hard to describe. If I know something is important to me, and not doing it is negatively affecting my financial, educational, or career situations, why do I ignore it and shrug it off? Most people with depression are acutely aware that what they do in life is negatively affecting them. But the disease pretty much drains you of your will and energy to do anything.
It's really hard to accept for most people, the person with it as well. I'm extremely self-critical. Forgetting a small thing in my routine, even if it's unimportant, will make me criticize myself more than necessary.
Cont.