Look through your piece and mark the tricky passages with a pencil. Next break these passages down into smaller sections. For example, a long run of semi-quavers can be broken down into groups of 4 semi-quavers, tricky intervals can be broken down into a collection of simpler ones. Mark these small sections with a pencil.
For the first half of the week practise only these small sections. For the second half of the week put these smaller sections back together and practise as a whole passage.
Get the small sections correct and you will find the formerly tricky passages much easier to play.