I think that a lot of writers get that feeling when they've been away from writing for a while. I get anxious, because I miss my characters and my stories, but at the same time, I find it hard to write. My solution is usually to write some notes about the next section I'm working on, and that often helps me get going (but not always). I hope you find what works for you. I've been thinking about this story kind of often recently, and I wish you luck in your work on it.
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I hope it helps! I think it is a little better than working on backstory or editing, since both of those can keep you from doing any new writing. Of course, they both need to be done, but I usually try to wait until I've finished a new chapter before going back to work on editing. Backstory... well, I usually try to have most of it done, but there always seems like there is more that I could do.