Clearly, happiness does not seem to correlate entirely with how well off you are.

Perhaps happiness relates mostly to social relationships, and spending your most productive hours with colleagues with whom you generally have relatively weak bonds depletes your 'social happiness' reserves.

Or perhaps the kinds of jobs many of us do in modern life, especially those involving computer screens and office buildings, are just not well-suited to us.

Or maybe it's just an inherent human property to be unhappy much of the time, and perhaps some people suffer more from this than others (a happiness 'baseline').

I could go on, of course, but fortunately a lot of research has been conducted on this, and many very wise people throughout the ages have written extensively about this.

One thing that could help is to start researching 'happiness' or what you could call 'the good life', whether through philosophy or religion or psychology, if you haven't done so already. And I don't mean 'google for happiness and click through few articles', but rather really dive in. Treat it as a proper project. Take a particular philosophy/approach that appeals to you, and actually apply it. Properly. At least for a while. A lot of it might be crap, or just not for you, but some of it will resonate. For me, it was Alan Watts (and zen buddhism), Seligman/positive psychology, and just the act of reading 'denser' philosophical stuff in itself that somehow tickled my happy bone (hmm, poor choice of words).

Also consider what you've tried, and most importantly, if you've actually tried these things. It took me until my thirties to realize that I had not, in fact, experimented with different ways/perspectives/philosophies of life in my twenties, but rather I had mostly half-heartedly flirted with all these things from a safe distance, and mostly let things happen to me. And all that generally just confirmed what already was while making me feel like I'd explored.

Or, to make an analogy: if you want to find a partner to be happy with long-term, this usually requires a degree of 'experimentation', as few people are lucky enough to find such a partner right away. Common mistakes in this regard are to 1) keep dating the same essential person, and then wondering why it keeps going wrong, or 2) be too shy to take initiative, and wait for things to 'happen'. I've noticed that 2 often ends up leading to 1, as you tend to attract and feel attracted to the same types of people if you remain the same kind of person.

Basically: break your patterns, and break them properly. Our natural instinct is not to, and cling to safety and the familiar. And if breaking patters becomes a pattern, well, then break that and go do the same thing with full abandon for a while.

If quitting your job seems like too much, and perhaps it is, then start with other stuff. Perhaps that will be enough. But just make sure that you really did try other things, because I think the illusion of doing or having done so is an easy trap to fall into. I personally did quit my job, because it was the only way for me to actually try different stuff, but then I'm perhaps just generally too inclined to get sucked up in what I do for work. You might not need this.

And finally, with my rather biased current perspective: give up on trying altogether. Just let yourself be 'now'. Perhaps read some of Alan Watts' material before you do this, so you have an idea as to why this might be effective.

For me the major breakthroughs and a more consistent sense of happiness came as a direct result of giving up (mostly) on all that finding. I can strongly recommend Alan Watts' 'The Book', as well as 'The Wisdom of Insecurity'. Regardless of whether you agree with him or not, he's an entertaining writer, and the approach he suggests is (relatively) unusual, perhaps especially to many of us HN types (ambitious, stuck in the 'virtual', analytic).