Next bit of Required Reading for worldbuilding is also by Jared Diamond (seriously, worldbuilders, this man is a goldmine), namely Collapse. While GG&S is about how and why civilizations prevail, grow and dominate, Collapse is about how and why they fail and fall.
One of the most important lessons from this book is how the prime reason for a civilization to fall is due to resource management failure, and this can happen to just about every civilization. And does. I love how Diamond eviscerates the myth that various non-European/Caucasian cultures are these gentle guardians of their environment, while Western cultures are destructive wasters. For example the whole "Native Americans used every part of the buffalo" thing. Yes, they did. And westerners used every part of the cows and sheep, too. And they pretty much still do on an industrial level. It's really only now in the modern, household usage that we throw away potentially useful animal products.
But the other underlying reason for a society to collapse, and for poor resource management, comes back to the Tragedy of the Commons. This is the fancy name for the idea that for any public resource, it is in the best interest of any individual to A. use as much of it as possible and B. invest nothing in maintaining it. Problem is, if everyone does that, the resource gets used up. But this happens all the time, and societies have fallen apart for it.
So in worldbuilding, look at how your societies use their resources, and if that is leading towards collapse. And the conditions that led up to that.
In my various Maradaine books, you'll catch mention of a country called Poasia. Mostly it's in passing, as Poasia had a war with Druthal in the recent past. Poasia is a nation that is potentially headed towards collapse. In my initial notes, made years ago, I had made it that Poasia never had a strong agricultural base. I've since revised that, based on last week's information. Now its that the resource base is dwindling, and the Poasians have spent decades overmining, overfarming, and pushing the limits of their resources and people. They mostly ended the war with Druthal because they were literally unable to maintain it.
Collapse isn't just about the failures, but also the success stories. Civilizations that excelled at resource management, and how they thrived because of it. Both Japan and Germany treated their lumber industry like long-term farming. Those same principles I put into place in Druthal, and helped define the character of the Druth people.
I'm still thinking about how to apply these principles on an interstellar level. Space Opera is quite fond of "Old Ones", ancient interstellar societies that eventually fall and leave only remnants. It's fascinating to think about, but I'm still trying to figure out how to make it work practically.