It doesn't take long for most writers searching for community and advice online to bump into Nathan Bransford's site. If you're not subscribing to his newsletter, you absolutely should.
Recently, Bransford posted an article on knowing when to stop writing (when it's time to stop revising and let a book a go). The section on recognizing problem novels is especially helpful. It all comes down to finding joy in your work. Easier said than done, sometimes.
A few ideas for reclaiming joy in your writing:
Holly Schindler - Recently, I've been gravitating toward humor in my work. It's become one of my favorite ways to explore who a character is. What a person finds funny says so much about who they are. Even the most serious of stories have room for humor--banter in dialogue, for example. If I find that a WIP is becoming something of a slog to write, it helps to insert some humor. Just the insertion of some banter can lift my spirits, put a smile on my face. If I'm smiling, the tone of the work changes completely. It becomes fun again. It's play. I want to roll up my sleeves and slide the pieces of a manuscript around.
John Clark - I have a trail of unedited and incomplete novels haunting me. 9/11 killed my writing ability for a couple of years, then back issues made sitting for any length of time agony. I'd get part way through a book and the dreaded slog descended, filling my brain with discomfort. Eventually, it became easier to quit than persevere. I was also seduced by my sister introducing me to short story competitions. Let's be honest, knocking off a piece of writing in a couple days and getting it published a month later beats the heck out of spending months creating a 90,000 word monster. What happened to change this? Mortality is one factor. I don't have thirty years left to dawdle and expect magic to do the grunt work. Another, oddly enough, is a magic candle I bought at one of my favorite stores, Enchantments, in Boothbay Harbor. Perhaps the most important is a fresh sense of confidence in my ability. In the past seven months I've written a new book, added two more stories to a YA anthology that's close to prime time, and am now editing the second book in a five book series about a teen with an extra finger and two girlfriends half a universe apart. For once, editing, and the thought of finishing those books that remain incomplete are things I'm anticipating and the dread is in my rear view mirror.
Mary Strand - Ironic timing, since recent events have crushed my joy in writing! But I tend to equate joy with action in novels, so when all else fails, I blow something up or at least cause a major accident to a character. And you think I'm kidding!
Sydney Salter - Sometimes to capture the joy of writing you need to write something just for yourself. Not for that often elusive market. I once wrote a series of short stories for no reason than to find joy in the process. I didn't share them with anyone, I simply enjoyed making up a bunch of stories about a bunch of wacky people.