Thinking back to when I was much younger, I had this notional idea in my head about what it would mean to be an adult. Looking back now over a few decades of having been an adult, it's easy to see where things just haven't lived up to the vision I had as a child/young adult. Here are five specific examples:
Myth - You can do whatever you want.
Reality - You can't; by and large you don't want to anyway.
Being able to do "whatever you want" doesn't mean you can actually do whatever you want. For me, I think the biggest obstacle in adulthood is energy: By the time I do all the things I need to do, such as my professional life, home responsibilities, etc., I basically don't have the time or the energy to do a lot of extra stuff. If anything, I have to work really hard in order to be able to do at least some of the things I enjoy. Sadly, sometimes it's easy for me to forget just what some of these things actually are.
Myth - Earning money makes your life far better.
Reality - Your overhead is considerably more than you ever imagined.
As a child (like most children) I only chose to see the having side of money. Rightfully so, I didn't have much of a window into the other side of the financial equation, namely just how expensive life truly can be, especially when young children are involved. Having to worry about paying bills is a kind of cancer that eats away at someone. I've known both the worrying and the "I'm doing okay" side of things, and one is definitely better than the other.
Myth - School is boring and stupid; it will be far better to have a job.
Reality - School is a far, far easier gig than having a job.
This one is a slam dunk. School was far, far easier for me than being in the working world has ever been. Even my least challenging job was harder than my most difficult school experiences. I want to note though that there is a corollary of sorts to this: At least some of the work stuff has been far more rewarding than school. This also loudly makes the case for adults to try and find "work" that they actually enjoy. With me, well, I know what I enjoy, but I just have to find a way to do it more often.
Myth - "All I want is for someone to love me and it will all be okay."
Reality - People...and relationship...are far more complex than you think.
Young adults are such idealized romantics, and isn't that a wonderful thing? Actually finding someone that you want to spend the result of your life with though is hard work, and for some, it should be even harder, because getting into a relationship can be far more difficult than getting out of one. It's a kind of catch 22 actually: When we are younger, at least some of us are in a hurry to find someone to spend the rest of our lives with, but yet that's when we are the least experienced with relationship and basic self-knowledge (we don't really know what we want ourselves in a relationship, let alone be able to articulate it to someone else).
Myth - Our bodies are indestructible and we can do whatever we want.
Reality - Our bodies tend to wear like car tires.
I never would have imagined how hard just living can be on the human body. What's more, and in retrospect, I could have never conceived just how corrosive something like stress can be to my physical health. It's nothing short of astounding. As a younger person, I more or less took my health for granted. These days? Well, I can't. Like some of the other points noted above, getting older means having to work harder (in this case, at my physical health).