A great deal is said, and a great deal deserves to be said, about the importance of perseverance in a writer's life. Some books need more than others (spoken with an extremely pointed glare at my work in progress), but it is a trait necessary at all times in the life of a writer, and, for that matter, in anyone's life; giving up at the first difficulty may be more common in our society, but it doesn't help in completing a novel.
At the same time, however, there is a place for moving on. I won't necessarily say that this point is ever reached because of difficulties alone, but there are other reasons for abandoning a work and continuing on to something else. One reason is stagnation. I spent about a year, and possible more, writing a mystery thingy (thingy is really the best description I can give it, looking back on it) that was very near to my heart, although my actual writing of it was somewhat sporadic and I didn't get far. Still, I toiled over it with great perseverance for a long while...and I believe I can safely say that my writing did not improve a jot throughout that time. It was not until I broke away from the Thing and began working on little bits of an incomplete story based around Stonehenge that my writing actually began to develop. Stonehenge was never a tight novel, per se, and likely never shall be, but it was a stepping stone, and after a while I deleted the Thing forever and moved on to write The Soldier's Cross.
Staying with a story is commendable, but there comes a time, especially if the novel is an early one or even a first, when starting something new is advisable. The comfort zone of the old has to be left for something different, and likely not as comfortable at first, if one's writing is going to progress. The same thing goes for such endeavors as fanfiction, which is another good experiment in writing to start off on, but which should give way to original works at some point.
Another time for moving on is when a novel is finished. In theory it doesn't sound as though this would be so hard - after all, the novel is finished - but it has its difficulties as well. Once a writer reaches the last page of a story he has been working on for months or perhaps years, there is a bond between him and the characters, and the novel has usually become comfortable for the author to work with. Thus it becomes quite easy to keep rewriting and editing and rewriting and editing, rather than starting work on another novel. (This is, of course, not to say that editing is bad; it is quite necessary, but can be taken to extremes.) Not being a fan of editing in general, I can't say that I would rather be editing Wordcrafter than working on anything else, but compared to the difficulties of The White Sail's Shaking, it doesn't sound like such a bad idea...
Attachment to a novel can also lead to series. Not the sort of series that are basically one storyline cut up into several books, but the Nancy Drew or Boxcar Children series that just. won't. die. This would be Disney, who makes one movie, sees its success, and promptly follows it with a sequel or two. This is a way of "moving on," since the writer is leaving one novel for another, but it can easily be as stagnating as staying with one story. The characters become so much a fixation that developing any others is more and more difficult - perhaps impossible - and the plots are often so familiar to the writer that they never bother to break out of the mold. Change, even for those of us who are not fond of it, is healthy.