If it helps there are some interesting papers on the idea that motivation comes from seeing the results of your own actions, and not before.

So that means you are supposed to feel this lack of motivation, and once you start writing and the feedback loop kicks in, the motivation will come on its own.

Feel free to ignore me if that doesn't help but here's a link to some more information on the idea: https://www.entrepreneur.com/living/dont-wait-for-motivation-do-this-instead/232349