You're right, but just to point out the obvious - you can decide (to some extent) not to be bitter, even if you aren't fulfilled at your job.
Many people, in fact most people, and certainly most people throughout history, have worked not because it was the thing they wanted to do to be most fulfilled. They worked because they had to provide for their family.
The choice isn't binary - it's not "do something you love and not be bitter" vs "work to provide and be bitter". You can also choose "work to provide, understand the tradeoff you're making, and accept it". You can still have a plenty fulfilling life outside of your job - many people do.
On the other hand, very few jobs these days in the West are about survival anymore. It's true to some extent that you need to pay for your mortgage but at the same time the amount of bullshit jobs is staggering. People work for work for organizations that provide the service of allowing others to like photos of your holidays. Very few people are employed in farming these days as opposed to the entertainment industry. I'm not saying that working in the gaming industry is without value but compared to historical data it looks like it's very hard to die of a famine these days.
This is me, I couldn't care less about massive multinational corporation employing me, it could go bankrupt tomorrow for all I care. But there are those annoying bills, mortgages, vacations, hobbies or just saving for retirement. Remove salary and I will stop working for my employer immediately. I am also not breaking my back to have stellar career, paycheck is enough. I did it for 20 years for various employers, can handle doing it for another 20 till (hopefully earlish) retirement.
But am I depressed? Not at all, because all the free time I get (which often included rather long lunch breaks spent ie running or weightlifting) I try to fill with family and my passions (which are not yet fully compatible things so its often either-or). Its mindset (doing work is OK, it doesn't have to be super exciting) and those passions. But it took me half a decade to discover them, most are adrenaline/extreme sports. TBH I don't know that many middle aged people with real fulfilling passions, that may be the source of problems.
Many people, in fact most people, and certainly most people throughout history, have worked not because it was the thing they wanted to do to be most fulfilled. They worked because they had to provide for their family.
The choice isn't binary - it's not "do something you love and not be bitter" vs "work to provide and be bitter". You can also choose "work to provide, understand the tradeoff you're making, and accept it". You can still have a plenty fulfilling life outside of your job - many people do.