Try a break, rather than another edit.
We've all been there: Rereading a manuscript and wondering if we should rework the beginning, deleting this backstory, shortening that dialogue, including more imagery...
The answer, often, is yes. It's essential to produce your best work before submitting to publishers, agents and competitions. Taking the time - once or twice - to check that your finished manuscript really is your finished manuscript is time well spent.
But at some stage, these rereads and edits can become counter productive. Sometimes we're just changing things, not making them better. Deleting a sentence here, adding a bit of dialogue there, deciding that a protagonist needs to be more or less assertive, that she needs to have a cat instead of a dog... none of these things will make a difference to the story.
What's more, it's incredibly frustrating to come across an earlier version of a 'finished' novel and realise it's so much better than the version you're currently editing. (I once spent weeks changing a 300-page manuscript from 1st to 3rd person, only to change it back again. To make it worse, I hadn't saved the original.) By constantly changing details, by taking on board every comment from early readers, by wanting to please everyone at every turn, all you're doing is delaying sending out your script and having agents and publishers read it. And yes, that's scary, but when you started writing your novel, wasn't that your goal? Everyone isn't going to like your novel because it's impossible to write a novel that would please everyone. So, ask yourself, if you think it's good, why are you really doing a fourteenth rewrite of your ending.
Constantly looking at a piece of work means you can't at all judge it as a first-time reader (agent, publisher, competition judge) will. You're not astonished by that perfect turn of phrase or amused by the sparks that fly between your characters. You can't be surprised by a plot twist or misled by a red herring. You can't loose yourself in that beautiful bit of narrative because you remember the effort it took to find each and every word. But your reader is reading it all for the first time and will be astonished, amused, surprised and mislead. If you give them the chance to read it.
Instead of tinkering for the umpteenth time, take a breather. Put your work to one side and leave it there for a few weeks. Work on something entirely different. And then, come back to it with fresh eyes. After a short break like this, it's a lovely surprise to discover how finished something already is and how what you've written not only works, but is good - exactly as it is. Paragraphs that didn't seem to work, now do. Scenes that were considered too lengthy, are the right length. Your protagonist's voice is loud and clear. You've saved yourself time and a whole lot of frustration. Now, all you need to do is submit.