All of my income comes from freelance writing, and I struggle with exactly the issue you have described. Even though I usually have a topic given to me by a client beforehand, I often find it hard to even come up with an outline of the topic if I am distracted by other things and just don't feel like it.

Sometimes I respond by trying to push through and work anyway. Those are the times when my work ends up not being as good. Other times, I just take the day off and go back to work the next day. But if I do that every time, I'll end up not making enough money to pay the bills.

I think you just have to figure it out on a case-by-case basis.

Thank you for writing this. I enjoyed reading it a lot.