OK, I posted the 'causes' comment and lots of cowards replied or moderated.
I have been there. I've been there with many others in a group situation.I know. The comment just above this is accurate for many- the rest are trash.
Many of my friends who shared this condition with me are long gone and forgotten. They died so long ago that they aren't listed on the internet. They are truly gone. The ones who survived did it by one simple technique: They learned to look outside themselves. They refused to turn inward.
What saved me was the Milky Way. It was still visible when I had my troubles. A vast infinite sea of objects, each little dot far bigger than I could even imagine. And beyond my vision many more. And here I am in a common type of galaxy, a boring solar system, a small blue planet, and I'm one of billions of creatures on that planet. Is my ego so big that I think I'm important? Not when I look at the heavens. I got over myself.
Some people may need drugs or other help initially. I can't speak for everyone, but I have observed many. But I strongly believe this to be true:
To the extent that you think you and your miserable life are important, you will suffer. Get over yourself, participate in the world around you. Join an organization that does good work. Read a book that isn't about sick people (I read every psychology book I could find at first). Exercise and learn tai chi. Help a child who is struggling in school.
And ignore anonymous cowards.