It's telling that Anne has to discontinue writing in shorthand due to her short-sightedness just as her mood is beginning to darken. Her world is becoming less clear, both literally and figuratively. Up until now, Anne has shown remarkable fortitude in the face of adversity, despite her tender years. But when all's said done she's still only a child, and inevitably there's only so much she can do to stay strong. From now on, the tone of her diary will become notably more world-weary and cynical.
It's ironic indeed that Anne is deprived of a much-needed pair of glasses as her parents think that the war will soon be over. Their wishful thinking, their figurative myopia, if you will, represents a kind of blurred vision of its own. The Franks are desperate for the war to be over as soon as possible, and so are prepared to seize on the slightest shred of evidence that they will soon be delivered from their current ordeal. So Anne must do without her glasses and this means giving up writing in shorthand.