I agree with lisa, whether you're depressed or just down depends on how long you have been this way & how bad it is & maybe on why.
So yes, try writing down all your feelings, - what makes you feel good & what makes you feel bad or sad? What leads you to think about the world without you in it? & how you think this - I mean is it, have I donme anything worthwhile or is it the world/my family would be better off without me.
When did this start & what triggered it, if you know or was it a series of things??