Depression

It's a feeling of sadness and sorrow, like no one cares about you and that you can die and they won't even notice it. It sometimes leads to suicide, and sometimes subsides with simple stuff, like getting a new phone. Depression is one of the worst cases of mental issues.

"My depression is the worst possible thing. It feels like it's eating up my soul."
"I hate life. Depression and suicide is all I ever think about nowadays."