For any writer struggling to write that second book, or any book, look at this blog post from Tess Gerritsen. I spotted it on Marjorie Liu's blog.
Writer doubts. Everyone has them. First drafts are crap drafts, revise, revise, revise. Then revise again.
I've discovered that if a character begins acting a certain way, there's a reason for it. Cut them loose. Let them wander but give them a road map as a guideline. In the end, you must be true to your muse. Stop telling yourself it's crap. It's not. It's called the writing process. And everyone does it differently.