When working out a difficult selection during practice, first break it down in to sections and agree on fingerings (both LH and RH). Once comfortable with all sections, slowly start putting them together to a time reference. The most difficult section is your terminal tempo. In other words rehearse NOTHING faster than you can perform the most problematic portion. Then gradually increase to tempo with all sections in tact; otherwise, your ‘problem’ section will forever remain- ‘a problem section’.