But she never said it was easy!
In fact, while posting on Twitter (as she regularly does), JK admitted that there was one chapter in particular that nearly broke her.
Chapter 9 of Goblet of Fire.
"Chapter 9, Goblet of Fire nearly finished me. I rewrote it more times than I can now remember. You'll get there," she wrote, encouraging a struggling writer.
The chapter she is talking about to in her tweet from the chapter called "The Dark Mark."
In it, Harry, Hermione, and the Weasleys go to the Quidditch World Cup. After the match, Death Eaters attack the campsite and the Muggles who live there. It's Harry's, and us as a readers, first glance at what the wizarding war is actually like. From there, the tone of the story delves much darker, so we're not surprised that it was such a challenging chapter to write.
However, JK also added that "perfection" shouldn't be the goal, because it's just impossible to achieve. She wrote in response to a fan who called her work "flawless," that the series was created "certainly not flawlessly, but I did it. Never wait in expectation of perfection or you'll wait forever. Do the best you can with what you've got and be one of those who dared rather than those who merely dream."