I think I sorta disagree with you if I understand what you are saying. You can be happy without a fulfilling career but the ultimate happiness is if you have a career that defines who you are and makes you happy. Where I disagree with what "they" say is that you have to find it right away. I was on my third career and my third job within that third career before I found it. It is nice knowing that I'm under forty and I'm going to retire from the job I have now. It took 7 jobs, post degree, to find it but I finally did and it's amazing.
if you have a job that you love, that's fantastic. i've seen people like that, i know people that have found that situation. it's an enviable position in life. what disagree with is placing this as the goal. i feel that for my first 25 years, people incessantly encouraged me to do what you love, find something you enjoy, and make it your job etc. looking back, it was foolish advice. i studied what i love, i have a job in that field. the work i actually do has little to do with the stuff i like about the field i chose because i liked it.
oth, if i'd chosen to do something based on compensation in that field, i could have done work equally as enjoyable as what i do, but i'd have the money to pursue other options not open to me now due my financial situation.
i think the situation i've found myself in is more common than the one you've found yourself in.
i'm not entirely happy with the above. i think it more or less describes my position in practical terms. but it misses something on the philosophy. a job is you selling your time for money. your time on this earth is a limited resource. you are a very rare human if you are able to sell your time for doing the exact same thing that you makes you happiest in the world. assuming you aren't one of those very rare humans, i think it is a mistake to focus too much on moving how enjoyable the way your sold time is spent a little closer to happiest, instead of focusing on selling control of as little of your time as is possible.