Have you ever noticed that when you tried playing that difficult passage, run or scale that you were holding your breath? Breathing provides us with the necessary energy to execute these physically strenuous tasks. By holding in the breath we are in turn working against our natural tendency to provide our bodies with energy and life force, the same elements that give us the strength and endurance to perform something that is difficult.
The next time you find yourself holding your breath while you are playing, relax yourself and open your mouth slightly (most likely you had your lips tightly clenched as a means of holding in your breath). By doing this you will then start the normal flow of breathing once again.
When playing a scale pattern breath in as you play the scale ascending and breath out as you descend. With enough practice this will become an automatic response everytime you play your scales.
Now let's apply this technique to that difficult passage you have been trying to get down. Divide the passage/run into two halves wherever you see the logical midway point. If the run involves ascending then descending, the midway would be the point at which it changes direction. If the run is only in one direction then the midway point is the place where the run is equally divided, or at a point where a difficult fingering happens, if it is around the center of the run.
Play the run breathing in for the first half and out for the second. At the selected midway point that you have determined, shift your breath from in-breath to out-breath.
If you put these techniques into daily practice as part of your playing as much as any other technique you work on, I am sure you will find what was once looked at as difficult will be much more manageable.