Often a student will choose a priority and then decide to spend several weeks concentrating on it. The result after a few days is that the player's work deteriorates. What has happened is the normal balance of the playing has been severely disturbed. The overall capability of even moderate players is more complex than can be imagined. Obsessive concentration on one aspect of playing wrecks this balance.
All players know more or less where they stand in relation to their own capability.
When that balance is disturbed, panic results. Frustration is created by the lack of fulfilment of unrealistic targets and ambitions. These will improve the background competence of the player's work and gradually filter through to performance. On my own single days off, when I didn't want to practise but had a tough piece the next day, I would leave three or four trumpets around the house. When I passed each of them I would play just one note, always a different one.