I jump up: it would be much better if I could only stop thinking. Thoughts are the dullest things. (22.34).
One of Antoine's biggest wishes is that he could learn to stop thinking altogether. After all, his thoughts aren't very cheery, and it's very difficult to not think about something that's bothering you.
I have no troubles, I have money like a capitalist, no boss, no wife, no children; I exist, that's all. And the trouble is so vague, so metaphysical that I am ashamed of it. (24.36)
One of the reasons Antoine spends so much time hanging out and thinking about existence is because he has all the money he needs to never work again. For this reason, he has trouble justifying his existence, since there's really no reason for him to get up in the morning apart from whatever reason he invents for himself.
I am in the midst of things, nameless things. (24.296)
One of the things that troubles Antoine most is the question of what kind of world exists beneath the one created by human language and human concepts. You, for example, might see a table and think, "table." But the truth is that you're staring at a bunch of dead physical material shaped like something you recognize. The matter, though, doesn't care what you think it is. It's just there and it could just as easily be a pile of wood as it could be a table.
The Nausea has not left me and I don't believe it will leave me soon; but I no longer have to bear it, it is no longer an illness or a passing fit: it is I. (25.1)
Antoine claims to make peace with his Nausea when he admits that the sickness is a fundamental part of who he is. For him, the feeling of Nausea is directly connected to his ability to think as a human being. The moment you start wondering about the meaning of your own existence, Nausea is where you're going to end up.
Every existing thing is born without reason, prolongs itself out of weakness and dies by chance. (25.12)
Whoa there, Antoine. You're starting to sound a little depressed. Apparently, things are born with no reason and they die out of total randomness. Well that's just peachy.
That little sense annoyed me: I could not understand it, even if I could have stayed leaned against the gate for a century; I had learned all I could know about existence. (25.15)
Eventually, Antoine becomes convinced that he has learned all he will ever know about existence. He knows that it's a completely indifferent thing and that humanity's struggle for meaning appears pathetic and absurd once you realize just how cold and uncaring the world of things actually is.
The Nausea has given me a short breathing spell. But I know it will come back again: it is my normal state. (31.5)
Every now and then, Antoine succeeds in forgetting about the pain of existence. But he knows that it will always come back. Nausea is a part of who he is. It's not something he can just get over like a common cold. To think is to feel Nausea. The very ability to care about the meaning of life is the same thing that makes him feel pain at the thought that life might be meaningless.
Existence is what I am afraid of. (31.8)
When it comes time to paraphrase what is bothering him so much, Antoine admits that he is afraid of "existence." In other words, he's afraid of the crushing truth that the universe doesn't care at all about humanity and never will. It's just a bunch of earth and rock that doesn't have feelings and doesn't believe in human progress or significance.
Now when I say "I," it seems hollow to me. I can't manage to feel myself very well, I am so forgotten. (33.3)
When Antoine uses the word "I" in his diary, he doesn't even know what it refers to. He knows that he is a person, but isn't sure what exactly he means when he says "I." He means himself, of course, but what does that mean. After all, there are billions of other people in the world who use the word "I," and they all mean something different when they say it. Think about it… Who are you? Who is "I"?
And I, too, wanted to be. That is all I wanted […] to drive existence out of me, to rid the passing moments of their fat, to twist them, dry them, harden, purify myself, harden myself. (33.48)
Toward the end of Nausea, Antoine claims that all he has really wanted out of life is to exist or be in some sort of authentic way. He wants to escape the pain of existence without distracting himself. He wants to face life directly in all its harshness and still find reason to live. He wants to be tougher and harder, stronger and faster. Unfortunately, it doesn't sound like he has succeeded.