Mid-Book Slump or Time to Abandon Ms?
As writers, we get ideas all the time. When we say writers block, we don't mean not getting 'any' ideas, we mean not getting any good ones. We need ideas that stick with us.
Have you ever had that feeling when you've started to write a book that this isn't the idea for you? Or caught on to a new idea that seems to outshine your current manuscript? How do you know whether it's simply your mid-book slump and just a distraction, or if the new idea is simply better than the current project?
Pursue Both Ideas... For a Little While
Write a few chapters of each. Maybe alternate. Whichever devotes a decent amount of time to both projects. You'll feel your heart calling you to the right one. (Though I have found mortal hearts to be rather fickle, but this is a good temporary test.)
Write Down the New Idea, then Forget about it... For Now
Sometimes the new ideas do simply distract us from a current project. Not to say they aren't wonderful and deserve attention, but maybe we can postpone that attention until you finish this project.
Determine What is Bothering You about Your Current Idea
Why does your current manuscript no longer seem as... shiny as it once did? Is it something critical about the manuscript? Are you not connecting with your MC like you thought? Is the plot not unfolding you like anticipated?
Sometimes those kind of problems, the large problems, can be fixed as you go. Don't like the voice? Change it from this chapter onward. It's a lot easier to go back after having written the manuscript than to make changes when you've only drafted a third of it.
If the problems can't be fixed, then maybe it is time to move on. Shelve the current idea. Maybe you'll think of a way to reconstruct it later.
If there aren't any gaping problems and you simply prefer the new one, then try your best to finish the manuscript. You could simply be in a slump. With the amount of hours you need to devote to drafting a novel, no one expects you to be joyfully sprinting that marathon all the time. Grit your teeth and finish it.
Most of All, Go with Your Gut
So there aren't any huge problems. You're a third of the way through writing this manuscript. You have another idea, and no matter how much work you put into your current project, it's the new one that keeps drawing your attention. Go for it. Your instincts always know best, in the end.