Writing is such a precarious thing.
Some days are naturally better than others, though there are usually some clear ingredients that make for good productivity. Concentration, focus, drive, passion… these are all things that help to spur you along. But I find that the best way to be productive is to lower the pressure.
Writing is meant to be fun. You’re meant to enjoy it, right? So don’t put too much pressure on yourself to get it done. It shouldn’t be a chore and you have to accept that some days your creative juices will flow better than others. I try not to be too hard on myself when I have a slow day, because your flow will always return, usually when you’re not thinking too much about it.
Being hard on yourself only stagnates the flow. Writing a book shouldn’t feel like you’re writing an academic essay, so relax and enjoy it! Self-doubt creates mental barriers and writer’s block, so be confident and stride onward. Yes, some writing days are slower than others, but at least you’ll be making progress. Every word is progress.
However, I’ve come to learn that some days are quite simply a lost cause. So you have to learn when it is worth sailing against the wind or abandoning ship entirely. Of course, it is usually worth sticking around, just in case the wind catches your sails!