Why the Progress You Make in the Practice Room Seems to Disappear Overnight

Have you ever been frustrated by the fact that you can take a difficult passage, work on it for a bit, get it sounding pretty good, but return to the practice room the next day to discover that you're back at square 1? That nothing has really changed? And despite how good it sounded yesterday, now it sounds just as bad as it did before you worked on it? Most of us can live with "two steps forward, one step back." It's the "two steps forward, two steps back" that makes us want to tear our hair...