Well, obviously it doesn't always turn out that way. One of the most grownup things I've ever done is realize this, learn to be ok with it, and accept that mistakes aren't the end of the world.
For example, in my "previous life" with my ex-husband, we were both working full-time in fulfilling careers (albeit mine was contract work for non-profits, which was far from stable), we had a house, 2 vehicles, and 2 cats. We had accumulated all of the ear-marks of what people assume means someone is doing well and should be happy and grateful and waking up every day pooping rainbows.
Turns out, accumulating things other people want isn't enough. I was desperately unhappy (and likely clinically depressed, but was too ashamed to get any actual help until my ex and I separated). I blamed myself for failing to be happy, for being so selfish as to not want all these things I was so fortunate to have, for not being able to just adjust to the life I was living and just deal with it.
That's not to say I didn't make the best of it. I ran a writers' club, I made crafts and sold them at craft fairs, I visited friends out of town, I painted, I wrote, I did all the things that the creative part of me wanted and craved.
But it wasn't enough. For some reason it was never enough. And I was worried that I was just destined to be unhappy and that my fate was to be miserable despite all efforts.
Fast-forward almost 5 years later, and what a difference a divorce can make*. My partner and I are engaged, we have stable, well-paying jobs, one vehicle, 4 cats, a dog & a bird, an uncomfortable amount of debt, and we're going to be homeless in about a week because we haven't been able to secure a new apartment for when our lease runs out at the end of this month.
*your mileage may vary. My advice to people having a hard time isn't simply "get a divorce". Now, please, put the lawsuits away.
But it's ok. I'm pretty darned good, actually. I'm not panicked, in large part because we have places to stay temporarily until we do find an apartment. I don't long for the stability of my first marriage because, even though the stresses were different (for example, I didn't have the debt hanging over me that I do now), now they feel so much more manageable. Now they feel conquerable. Now I don't feel resigned by my life, but rather excited to live it.
So, adultivity be damned. The expectations of living a particular kind of life by a particular time frame may work for some, but it's not a one-size-fits-all formula to happiness. It's ok if you don't want and/or aren't happy with the traditional goal-posts. And even if it's what you want, there may be setbacks that take away some of your hard-earned gains. That doesn't mean you've failed; it means you're not impervious to the vicissitudes of life.
And if you fail? Well, that happens, too. There may not be a reset button with life, and some failures you may never be able to take back, but that doesn't mean you can't go forward
If you are struggling and feel like you can't get your head above water, please seek out help. Whether it's confiding your issues in your partner or a friend or a counsellor (goodness bless college-paid campus counsellors) or your religious leader or your parents or your cat, it may help you feel a lot less lonely and help put things in perspective. There's no shame in not being able to "fix" yourself or solve all your problems without assistance, sometimes including medical or otherwise professional assistance.
These meandering, trite plattitudes brought to you by Monday-afternoon introspection.