There is a difference between sadness and depression. Sadness you have for a day and you get over it. Depression can go on for years. Imagine feeling nothing for three years.
Imagine having to force yourself to eat because you don't have the desire or energy to.
Imagine not having the energy to do anything even stuff that you used to enjoy.
Imagine not having emotions.
Sound fun?
I didn't think so.
Imagine someone telling you "there's nothing wrong with you" when you ask for help.
That was like a slap in the face.
Not all illnesses are visible folks. Remember that.