I've discovered that writing a book is a little bit like childbirth: you forget exactly how, erm, uncomfortable the whole thing was until you're in the midst of doing it again. And then it's like, "Oh right. That hurt like shit. I remember now." By that point you've got no choice but to press on.
Writing a sequel is easier in obvious ways: you know the characters and you know the backstory, so many of the facts going forward are already set, and the reactions easier to anticipate. Conversely, this is also the hard part. You can't change or rearrange things for story flow. Sometimes I find myself feeling a bit like one of Cinderella's sisters, trying to shove a tiny slipper on a big fat foot. Why. Won't. This. FIT?!?
I have a complete story arc this time, as well, which is odd for me. I know what you're thinking: How odd can it be? You've only written one book. But that's not true. See, most authors have possibly a few books that are locked away in a dust-covered box that shall never see the light of day, and I'm no exception. Except that mine are on a usb stick in a drawer somewhere. And no, you can't see them. (Seriously, you don't want to see them.)