I suspect it's the same for all writers - you've written the last chapter, revised the novel, got advice from trusted beta readers, tweaked and polished, until it's the best you can do. It's finished; now is the time to move on to the next book. But you don't want to. You want to stick around with those characters you have grown to love - even the nasty ones. They're closer to you than anyone real, and you can't bear to let them go.
Of course, you can side-step the issue by writing the sequel - but if you haven't yet sold the first novel, this is a Seriously Bad Idea. You could waste a year writing a book whose fate is inextricably tied to the first book, and may sink with it. It's time for something completely different. Which is what I started doing today.
I got the germ of the idea for Untitled on the long cold walks between home and workshop I've been doing since I fractured my humerus, and can't ride my bike. I brooded about how my broken bone had diverted, I hope briefly, the course of my life; I noticed all the useful stuff Londoners throw out; and I trespassed inside an unfinished, abandoned and vandalized block of flats on my way home.
I've been typing notes for a couple of weeks, but today I began writing a scene a few chapters in where my heroine meets a man who will play a large part in the plot, and I think the transition has been made. I'm beginning to find my new characters as absorbing as the ones I've regretfully left behind in
Heart of Rock.
Woohoo.