I too have experienced this existential dread, and sometimes still do.
Your post triggered a multitude of responses from different people experiencing the same, each with their own set of beliefs and coping mechanisms.
I found it fascinating - thank you.
It always surprises me that we are in a minority - most people go through life without experiencing this, or wondering about this. When trying to discuss this with one of them, they are usually baffled, and truly cannot understand you. It is almost as if we're speaking different languages.
As a side note - I am wondering if there is something in common in all those who experience this dread.