If you're not familiar with user story mapping, it is something like this: first you think about the high-level functions and features you're going to be delivering and you prioritize that horizontally from left to right. Then you take your user stories for each of those features and you prioritize them top to bottom. Here's a picture to help you see it.
What I had seen with my customer was that we had gotten too focused on one of the features and spent almost a whole iteration getting it perfect, rather than prioritizing all the stories across all the features. By setting up a story map they could see all the work that still had to be done, which help them decide on what stories were really needed for the first release and which were just window dressing.
There's this idea of the walking skeleton, which is the bare-bones minimum that you could include into a release and still have it functional. Story mapping helps you identify the walking skeleton because you're looking across all the features. Going back to our picture, we would draw a horizontal line through the user stories. Everything above the line is part of the walking skeleton and would be delivered in the first release with the stories below the line coming in future releases.
So if you're having trouble prioritizing your stories try using the story mapping approach to help you get the big picture.