Go to the doctor. Seriously. Go.
I think I'm depressed. I don't want to go to the doctor's though, in case I'm just feeling really down or sad, so I don't want to waste the doctor's time. What should I do?
If you think you are depressed go to the doctor. You are not wasting his time, he gets paid for seeing you. If it depression, he can give you medication. If not, at least you will know:)