I stand up, I look out my window at the big city. I think 'how is any of this real, and why do I have to age and eventually die?'
I wake up in the morning in my weird little flat. I wonder to myself, 'so this is it, huh? I just do this until my body fails?'
I cook myself a meal. I find out how a museum works behind the scenes. I get a tour of an office. I see my friends go out. I book a movie ticket. I work out. I watch a comedian. I listen to a podcast. All of these things just make me ponder what the point of it all is. Am I doing it wrong? Am I doing it pretty well? Why should I accumulate all this knowledge if I'm just going to die? What's the point in watching my stupid obscure movies that I can't even talk to people about? Am I missing out on the human experience?
Realistically I'm a happy ape. All my needs are satisfied. But I am a sad human.
I think it's all linked to graduating. The pressures on to do well, the workload is racking up, and then once that's all done I just get thrown into the real world. That's it. Then it truly is just doing the same thing over and over. Then it really is a question of survival. Would it be better to just be a dumb neolithic huntsman who is grateful for his bed of fur in his cave? What the fuck did those guys even think of when they took psychedelics? Well, spirits, I know, but wow, the things in their head must've been so original. If I do them, most of my thoughts are just about the garbage I've watched, and my modern worries that are worrisome but relatively tame.
Is this just a normal thing to go through, and then you get on with it and accept that this is just how things are?
Maybe life is simply starting to get to me, and time will tell if I crack under pressure.
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.