Yes, finding a good title can be difficult. Like many others here have said, sometimes the title exists even before you write your first page, but often it's the last thing you'll work on after finishing the manuscript. I've have both experiences.
Re-reading your manuscript to find a phrase that would make a good title is a good idea. You may also want to take a large piece of paper and just write down any words or phrases that come to mind when you think about your story, including names. Sometimes, a good title can be created by merging some of these ideas and words.
Someone here has suggested checking the titles of the books you like and how they relate to the story. That's a good idea, too, especially when you take a moment to think about why a particular title appeals to you or intrigues you.
I often prefer short titles which somehow suggest the theme or the mood of the story (my screenplays include The Muse, Sleepless and Hold Your Breath).