This is also answered on the FAQ page, but I would say dealing with how solitary an occupation it is. You can get very lonely and buried in your own head. The hardest part of writing the Mortal Instruments series in specific was teaching myself how to write a novel. I’d never written a full-length novel before. I had to learn the structure of it, and how it worked. The best thing about being done with the first book and starting on the second was thinking, “I’ve done this once now. I can do it again.”