Thanks for the question!
Existential depression happens when we get disconnected from our spiritual side, and our true purpose in life, and from God (sorry, I said the 'G' word...)
When life is framed in term of material achievement, then it can be precisely when we achieve that materialistic pinnacle of success that we suddenly realize that it's not all it's cracked up to be, and it didn't really solve our problems.
That's why so many of the materially 'successful' people in life commit suicide, go mad, or retreat into substance abuse (sometimes covertly, other times in a very obvious way.)
They won the lottery - and it still didn't take away that gnawing feeling that life is empty and meaningless.
So what does?
The following things:
By contrast, when we make an effort to decode the hints that God is sending us via all the different facets of our interactions and experiences, absolutely everything in life becomes meaningful, because we know it's a tailor-made message from God, about something we need to work on, change or fix.
When people live with the reality that this world is just a temporary corridor, and that the real successes and achievements are the internal ones - like overcoming things like our anger, jealousy, ingratitude and hatred (OK, I have many more things to work on, on my own list, but hey, I have to keep some of my dignity...) - THEN the existential depression finally disappears, even if the 'externals' are not so good, or even downright bad.
Check out my book 'The Happy Workshop: An 8 week journey to real, lasting happiness' - click HERE to buy it on Amazon.