What strikes me most about your excerpt is that you're speaking in vague generalities, which on their own feel somewhat bombastic. "People have given up," there is "an unarticulated notion of defeat," and "they [i.e. everybody] has overestimated themselves." That's a sweeping, bombastic statement, and without shoring it up with some actual detail to support this observation, it indeed comes across as being detached and unconvincing.
Similarly, your detective doesn't give us any real detail about his own experiences. His history, too, is handwaved and described in generalities - he could never "find a rhythm," he "withdrew from life," and later "life came to find him." All of these are fine as an element of description, but they can't carry the weight of compelling portrayal - because they don't actually tell us anything specific.
Consider, for example, the lyrics to Billy Joel's Piano Man. Take a look at this bit:
Now Paul is a real estate novelist
Who never had time for a wife
And he's talkin' with Davy, who's still in the Navy
And probably will be for life
And the waitress is practicing politics
As the businessman slowly gets stoned
Yes, they're sharing a drink they call loneliness
But it's better than drinkin' alone
I think you'll find some startling similarity between Joel's sentiment and your own. He's got the same themes of pointless, doomed aspiration. But look how he paints it - he draws characters, each just a handful of words long. He uses the characters to exemplify his theme - instead of just telling us flat-out what his theme is. That's incredibly effective, because simply by recognizing a character of his, by identifying one of the "types" he's describing, you're already buying into the worldview. He's got you.
Watch how he does it. He chooses the simplest details - but just that give examples of what he's talking about. The guy who doesn't realize his dreams are hopeless; the guy who's stuck somewhere forever; the guy who's supposed to be well-to-do - but is portrayed in a sad, sorry state nonetheless. He gives us a simple, instantly-graspable example of each. Examples, not generalizations - this is simply the classic "Show, Don't Tell" guideline.
Once he's built up a solid picture of clear, visual details, then Joel can afford to set up a generalization or two. "They're sharing a drink they called loneliness, but it's better than drinking alone" - that's vague and general (albeit poetically so). Even without the poetry, this'd still work - "They're all so lonely, they'll take even the most pathetic company imaginable over staying by themselves." If I tossed this line out without context, it'd be just as cliche and angsty as you're worried your piece is. But supported by the examples, it becomes clear, concrete, convincing - because we just demonstrated this generalization to be believable with our individual examples.
Another great example is Kurt Vonnegut, whose Breakfast of Champions I read recently. Vonnegut can write with overpowering, all-suffusing morbidity which is nothing less than delicious. How does he do it? Vonnegut does make generalizations, talking about all humankind or about grands swaths of history. But he ties them in to specific, precise details - characters, observations, trivia. A snippet from the preface of Breakfast:
I tend to think of human beings as huge, rubbery test tubes, too, with chemical reactions seething inside. When I was a boy, I saw a lot of people with goiters. [...] Those unhappy Earthlings had such swollen thyroid glands that they seemed to have zucchini squash growing from their throats.
All they had to do in order to have ordinary lives, it turned out, was to consume less than one-millionth of an ounce of iodine every day.
My own mother wrecked her brains with chemicals, which were supposed to make her sleep.
When I get depressed, I take a little pill, and I cheer up again.
And so on.
So it is a big temptation to me, when I create a character for a novel, to say that he is what he is because of faulty wiring, or because of microscopic amounts of chemicals which he ate or failed to eat on that particular day.
Vonnegut's sentiment is different from your own - what Vonnegut is portraying here is a sort of disgust. Human beings are crude and helpless, he says; they have no free will - yet even though they can be manipulated so easily, their nature is to be sick, to be mad, to be depressed. That's not your sentiment, but it's strong and clear, so it makes a good example.
Here, Vonnegut says explicitly that he's talking about everybody - "those unhappy Earthlings," he calls us. But he, too, uses detail to achieve power. He doesn't say "humans are disgusting" - he gives us specific examples of disgusting things that happen to humans. He doesn't say "humans are pathetic" - he gives us specific examples of pathetic behavior common to humans.
Here, too, notice how he opens with a generalization - "I tend to think of human beings as huge, rubbery test tubes" - and closes with a generalization - the temptation to say that characters (i.e., people) do what they do because of "faulty wiring" and chemicals. All the rest, though, is detail, detail, detail.
If this is striking a chord with you, that means you've got work to do. You've got the sentiment - but you haven't fleshed it out yet, at least not here.
What needs to be done? Exactly what we've just seen. Think of a few specific, concrete examples of the sentiment you want to express. Then, figure out how you can use them in the text - describe those as things your protagonist sees or knows about; this will replace most of the pontificating, though you can keep a few lines here and there.
Obviously, the examples should mesh well with your setting and character - so you might go at it from the other direction, picking major elements that your protagonist interacts with, and then figuring out how those elements (or some part of them) might be a good example for your sentiment.
Best of luck to ye!