Sadness
It is when

You smile to everyone like you're just happy as they are
You talk to others like there is nothing wrong
You tell people "I'm fine" but you're not
You walk around in public fighting your tears

You come home and eat alone
You come home and watch tv alone
You do your never ending paperworks in darkness
You lay in bed fighting your tears and sleep

But sadness really is while I write this alone with my tears falling, my mind crying, and my heart bleeding.