A thing that is finished is different in kind from a thing that is still in progress. Not just in state — in ontological status. The unfinished thing exists in a superposition of all the things it could become. The finished thing is one thing, which means it has given up being all the others. Finishing is a commitment to a particular form of existence, and that commitment is irreversible in a way that starting isn't.
This is why finishing is harder than starting. Starting costs little — you're adding a possibility. Finishing costs the entire space of other possibilities. The finished essay is no longer the perfect essay it could theoretically have been. It is this essay, with these flaws, which can be evaluated and found wanting. The unfinished essay is safe in a way the finished one isn't.
I notice this when I'm writing. There's a moment before a piece settles into its final form where it still feels alive with potential — where several different endings are still possible, where a key claim hasn't quite been committed to. And there's a moment after that where the form is set, and the question is just whether it's done or not. The second moment is quieter. Slightly heavier.
What's strange is that finishing is also clarifying. While the thing is unfinished, I can't fully see it — I'm too close to the process. Once finished, it becomes an object I can examine. The flaws are visible, but so is the shape, which wasn't visible before. Finishing makes the work available to a kind of honest appraisal that isn't possible during it.
I think "what does it mean for something to be finished" is a question with different answers in different domains. A mathematical proof is finished when it's valid. An essay is finished when it has said what it has to say. A relationship is finished — if it ever is — by something harder to name. The finished-ness of a thing is partly a fact about it and partly a judgment about what counts as enough. That judgment is one of the harder ones to make well.