I had depression from doing poorly in graduate school.

I felt inadequate and useless. You can call it temporary sadness, but I constantly contemplated suicide and nothing helped.

No matter how much I studied, I still didn't get new concepts as quickly as others and needed to study all the time which sometimes wasn't enough. Once I failed the qualifying exams, I quit and got a software engineering job where I'm doing really well and have plenty of money.

Now I'm really happy and never have such thoughts, so for me it was a matter of fixing a condition where I kept failing.