In that brilliant talk on creativity by John Cleese, he mentions how creative people are almost defined by how long they’re prepared to suffer with an unsolved problem, as being stumped is a integral part of the process.
This article in the Guardian takes a related tack and is well worth a read. I loved this insight about left/right hemispheres:
The process began with an intense mental search as the left hemisphere started looking for answers in all the obvious places… This left-brain thought process, however, quickly got tiring – it took only a few seconds before the subject said he’d reached an impasse and couldn’t think of the right word. But these negative feelings are actually an essential part of the process because they signal that it’s time to try a new search strategy. Instead of relying on the literal associations of the left hemisphere, the brain needs to shift activity to the other side, to explore a more unexpected set of associations.