I think they call what you are describing "existential depression." This may be small comfort, but I've read that this is more common in people of above average intelligence.

Unfortunately, you can't think your way out of this. And you probably are prone to thinking because you are probably good at thinking.

One thing I would recommend to you is to expose yourself to high quality thinking by high quality thinkers . . . read. Try reading the work of others who have posed the very questions you are raising. You're in very good company.

You won't find a cure, but you may find something to be interested in.