I have this little theory about writing books. It's not based on anything factual or scientific. I've simply observed over time how writer friends of mine learn to move on from a specific book they're writing, or have written, or even published. First books, it seems, are the hardest to let go. I know because I've only barely begun to let go of my first book after seventeen years. Issues, I'm telling you. And it's not just that I think it's a problem when you can't let go of a book; it's that I think the more quickly you let go, the better writer you're becoming.
The reason I'm saying any of this is because I've noticed that after my sixth book - which was a beast to write, I'm telling you - has already been easier to let go than any of my other books. It was the hardest, emotionally, to write. I put more of myself into that book than any other book I've ever written. You'd think I would be completely 100% attached, right? Wrong. I've already begun to let go of that book. In comparison to my other books, it has felt like an easier process to finish that book and move on.
I'm not sure why this is because I don't think any writer learns to love their craft less the more they write. And I don't think stories become less important to a writer. I think it's a point we reach in a professional career, published or not, to step back and see things through a different lens. If you want to keep writing books and put yourself completely into them, I believe you have to let go of each previous one you've written. For instance, I couldn't let go of Monarch, even after it was published last fall. It took me months to let go of that book even after it was out there being read and reviewed by people. It hurt to keep holding on. I couldn't work on anything major, and decided to go back to my first book for major revisions (the one I was still holding onto so tightly). But Monarch was so close, and then I jumped into this other book I was still so attached to, and I became quite a bit of a mess, unable to write anything new for the longest time. Finally, I let go of Monarch. I'm not sure how I did it, but I did, and I was able to write new material again.
I don't really have any advice. This is just an observation I've had with my own work. I've also watched friends go through this, so I'm not alone. I don't think it's healthy to hold onto a book too tightly after a certain period. It's certainly important while you're writing. I won't argue with that!