So I can really only answer in my own limited sense - I haven't read much philosophy, just dabbled and lived.
For my own encounter with those doldrums, what I encountered that resonated with me was essentially Nietzsche's own exploration of the loss of the justifications for passionate living that we are given by our environment, parents, religion, culture, etc - nihilism. It isn't a happy desert to wander through, but it's one that has potential. And then, of course, it's okay that whatever answers you find you may grow beyond and reveal to be another illusion. Nishitani's The Self-Overcoming of Nihilism was a good companion on that.
Also, The Illuminatus! Trilogy and Robert Anton Wilson's later books were a very important part of me acquiring a sense of humour about the collapse of my expectations.
Recently, reading Daoist philosophy, the notion of harmony with the vast systemic gyre of the universe and a similar dynamic ignorance of there being any particular, static, understanding seem to fit with this. But that's easy to say and hard to feel.