Depression sucks. Its all too common in modern society. I think there is too much emphasis on always trying to be happy though, its not natural. There is nothing wrong with having a positive attitude most of the time, lots of people need to find a safe outlet to wash away negativity.
Some people have pets, other people have great family support, and I think many people here have discovered cycling keeps their mind clear.
Get outside, even if it is just for a walk, nature has a unique calming effect.
A good book, or an enjoyable indoor hobby can also be helpful.
Set a few goals, and make sure you have a backup plan if you can't reach those goals.
Life can and does get in the way, mental and physical health should be the top priority.