"I don't know it well enough"

Have you ever caught yourself practicing more pieces that you know well and neglecting the ones that give you most trouble?

It's human nature - we try to avoid things that make us feel uncomfortable. And playing a piece that is problematic is uncomfortable all right - you don't feel the beauty of the music, you feel like a baby trying to walk and constantly falling down.

Here's what pros do - they are fine with being uncomfortable. Paradoxically, the more they do it, the more comfortable it feels.

All you need to do is to figure out where to put the uncomfortable so that you could do your work.