Once you’ve written your first draft, get away from it for a short bit. Some say a week; some say a month or two. I don’t know that the length of time matters as much as the distance itself.
When you’ve invested the time and effort into writing a story that your own mind created, it’s hardly surprising that you’re not objective about it. The story is great! The characters are great! Your working title is . . . yep, great! Everything is shooting out rainbows, unicorns are prancing across the page, and you can hear Morgan Freeman reading for your audiobook.
If you give it a little rest, though, you’ll find it’s easier to spot the things that need work. Something that sounds great at three a.m. often looks a bit less special by the light of day. Your manuscript is no different.
In the same way, if you’re struggling to make things “fit” in your story, walk away and focus on something else. If you have another story idea with characters in your head, saying, “Write me next,” then write up an outline and some basic points you want to cover. It might be exactly what you need to get the creativity on track once again. Absence makes the heart grow fonder, after all. You’ll go back to the main project ready to go with a fresh attitude.