There is such a thing as overworking your essay.
But when it’s your own writing, it’s hard to know when you’ve hit that point.
I experience it with my own writing too! It’s always easier to spot when it’s someone else’s writing and you have some objective distance from the first-hand drafting process.
With our applicants (and all applicants, it’s safe to say), there’s always a point when the essay has reached its natural peak and when additional edits are headed in the wrong direction. They reach the point of diminishing returns, and while continued editing might feel as if it’s soothing your nerves and your anxiety (is it really?), it’s actually doing yourself and your essay a disservice.
There are very few things I promise or guarantee during the process, but here’s one thing I do promise: the admissions outcome will not depend on using this or that magic word. It will not hinge on whether you wrote “effect” vs. “impact,” or whether you call your grandfather a “fisherman” or a “clam digger,” or whether you use the word “reputable” instead of “respected.” Keep it simple. There’s often more than one word that will be perfectly serviceable. Pick one of the good options and move on. If you’re not sure what a word means precisely, look it up. That will eliminate some of the words you’re considering in any given sentence.
What matters more than any given word is that (1) you know what you want to say in your essay and (2) you’re telling a story about yourself that’s meaningful to you and (3) it’s relevant to this particular audience/in this particular context. It must be coherent and well written. Well written does not mean you have to write at the level of someone who is applying for an MFA in fiction writing. The writing matters, but the substance is the really important part. Once you figure out what you want to say, you’ve already done at least half the work of a great essay.