I started in manual labor, moved to restaurants, then retail, now I'm in skilled labor as a bench technician / apprentice engineer. I've gone from homeless at 18 to having a studio all to myself in the most expensive tri-city area in the world at 31. I get what you're asking, and I feel qualified to chime in: It all boils down to what's called the "hedonic treadmill". It's a psych theory that a person's disposition is a combination of learned responses that is largely independent of outside circumstance. It's the reason that people who win the lottery don't usually feel like their actual quality of life has improved, and often wind up in a similar situation as they started. Same reason plenty of musicians and artists lead fulfilling lives and create brilliant works at a time in their life when they are living in total poverty on the fringes of society. Same reason a person who is prone to anger will always find a reason to be angry, even if everything is going great they will yell at a waiter for forgetting the onions as if the guy just fucked their wife up the ass.

While a person's means and resources are predictive (in a big scope) to what opportunities will be made available to them, that doesn't translate to how the individual feels emotionally at any given moment. Whether we are optimistic or pessimistic has nothing to do with how equipped we actually are to deal with life's hardships. It's totally counterintuitive that way.

Plenty of people suffer severe, suicidal depression; yet lead lives where they have the respect of their peers, widespread celebrity, and more money than they know what to do with.

Speaking for myself, I can clearly see my place on the hedonic treadmill. I've never felt a strong sense of purpose, for instance. Which is an easy feeling to understand when you're washing dishes in a fucking hamburger restaurant, but a little more surprising when you're working for a renowned design firm or a company that does advanced scientific research. That's just my disposition, the "depressed, angsty, existentialist" as you described it. It doesn't matter how many languages I can speak or how many instruments I can play or how much free time I have or how much money is in my bank account. I have my own "average" level of happiness, and I will return, approximately, to that average regardless. And it's weird, because this is probably true of everyone.