I can't disagree with your comment more. As someone who falls squarely in the Millennial Age range, late 20s to 30s (btw that's how old millennials are now), I find the issue with so many people in this position has less to do with their upbringing or personality in particular, and more to do with the fact that we are in many ways playing a game in which the rules have changed. And that's a direct consequence of the actions of not our parent but our parents' parents', who are somehow, still in power 20 years after they normally would have passed on control on the economy and government to the next generation.
And what I mean by that is, that those who have come of age over the past 20 years have spent pretty much their entire lives being read a recipe for success. Go to college -> get a degree -> get a job -> live a successful live. It's a simple step by step plan that every single person in our lives have reinforced ad nauseam, because for most of the 20th Century that has been the surest path to a happy middle class life.
The problem is that of course every single step in that path is now significantly more difficult than it was 20-30 years ago when our parents' generation did it. You can't just go to college anymore. It costs 10x what it did in the 80's and you can't even compare the cost to the 50's or 60's. It's also way more competitive getting into a top school. Even if you manage to get into college and pay for it, it's no longer good enough to just get a degree. You need to get a degree in the right thing and or find a way to differentiate yourself from the other 1 million students who graduate each year, because oh by the way our generation is the most well educated in the history of the world. Even if all those things go right finding a job is harder than ever. People retire later if at all now so the market is flooded not only with too many well educated applicants but the multitude of experienced candidates competing for the same entry level jobs. And even if you get a job, the crushing levels of student debt and the fact that wages haven't kept up with inflation, basically mean that it's incredibly hard to live a successful life on your own.
So basically the issue is that people have spent their entire lives calibrated for a set of conditions that no longer exists. There isn't a golden path to a cozy middle class lifestyle anymore. And once you start compounding that with the normal issues everyone runs into adjusting to an adult life you get this malaise. Everyone's poor and hates their life and social media is there so you can compare yourself to the 10% of the population who seem to have it all figured out either because they have rich parents to subsidize their life, or they got lucky and picked a major that actually pays decently, or they married their HS sweetheart and hate life somewhat less than everyone else.
And because it has to be said the fact that everyone keeps calling Millenial's lazy, and entitled for somehow not being as successful as previous generations despite the changing landscape doesn't help.