How can I tell depression and sadness apart?

[メンヘラ]

How can I tell depression and sadness apart?

 

[Skelly]

depression doesn't go away

 

[メンヘラ]

depression doesn't go away

Then I can't tell them apart since my mood feels always the same, a bit numb. Even when I feel happiness is kind of numb, even normal sadness and frustration are kind of numb as well.