johnreef knows what he is talking about. but what I like to do when I encounter these places is to play certain phrases in short bursts, taking a long rest between chunks of music. Do this for about 2 weeks- or however long it takes you to master the individual bursts of speed. Then, begin to shorten the length of rest between episodes. After a while, there will be no noticable break in the music for the audience, but in your mind, you are still playing shorts bursts, and it will be possible.
Which piece are you playing?
donjuan