words not coming

I finished my paper last week with a grand sprint to the end, and I was pleased with it. But I still have an essay for a collection due December 15, and I can't seem to get it done. It's not so much writer's block as a complete disinterest in the task. I have already written an essay that is very close to this topic, but it does not fit the topic of the collection. So I proposed a new essay that does fit in with the collection but does not require more extensive research. The paper has to be different because I am making a completely separate argument--it's not just a revision of the other paper. And I like this argument--I think it's smart and interesting and right, and my advisor does, too. As does the editor of the collection, apparently. So why is it that I don't want to write the essay? I think what is missing here is the excitement of delving so deeply into the text and discovering things there and developing my interpretation--I feel like I've already done all that. I know exactly what I'm going to say and exactly what evidence I'm going to use because I have spent a great deal of time with this text working on the other paper and this argument is sort of adjacent to that one. It seems like I should be able to spit out this essay in record time. I'm just not into it right now. And I'm not sure how I'm going to force myself over this hump.