You will never finish your manuscript. Unless you finish.
That probably sounds like a fortune cookie approach to writing advice, but the point is that there will always be more to do. More details, more editing, more tweaks, more dialogue or description that you could add or take away (I recommend taking away most frequently), and other endless changes. Here’s the thing to remember though if your aim is publication. You agent will have feedback. Your publisher will have feedback. The editor(s) will have feedback. And you’ll likely make changes based on all of it.
I don’t mean to say don’t polish your book. Definitely do that. Before you submit to an agent, you should do at least two editorial passes yourself. If you want to employ an editor before submission, the same applies unless you’re sharing your work elsewhere, like a writing group or with beta readers (not fam and friends), because self-editing misses a lot.
Finishing a first draft is something a lot of people never do. I know agents who’ve had people submit unfinished works and ask for the agent’s input or suggest someone else (a publisher even!) decide an ending. That will seriously never work! You have to finish your novel and then finish it again and again.
One reason people struggle to end their books is that they don’t have a plan. These folks are known as ‘pantsers’ as opposed to planners. They let the characters guide them. This approach can be pretty amazing when it works. If it works. But more often than not, these improvisational warriors create lovely characters with nothing to do. Pantsers I know have been able to save their work though by winging it for twenty or thirty thousand words and then sitting down to plan what will happen, sometimes quite generally, so that an actual plot arc occurs. I’ve read some really great, vivid books that took that approach.
Eventually, you have to buckle down and end your book. So that you can put it away for awhile (I recommend at least two weeks, preferably a month) and come back to it for that first edit. When you come back to it after being away, you’ll notice a lot more mistakes and you’ll appreciate a lot more of your triumphs. None of that can happen though if you don’t finish; if you go back to the beginning or the middle to ‘fix things’ before you just poop out an ending. Yep. I said ‘poop’ because it might be crappy. And you may well change it, perhaps dramatically in future edits. Please, please, please though – finish it. Someone out there is waiting for your story.