It seems so obvious that it actually feels insulting to point it out. But it's not obvious. Every adult I know--or at least the ones who are depressed--continually suffers from something like sticker shock (that is, when you go shopping for something for the first time and are shocked to find it costs way, way more than you thought). Only it's with effort. It's Effort Shock.
We have a vague idea in our head of the "price" of certain accomplishments, how difficult it should be to get a degree, or succeed at a job, or stay in shape, or raise a kid, or build a house. And that vague idea is almost always catastrophically wrong.
Accomplishing worthwhile things isn't just a little harder than people think; it's 10 or 20 times harder. Like losing weight. You make yourself miserable for six months and find yourself down a whopping four pounds. Let yourself go at a single all-you-can-eat buffet and you've gained it all back.