How could I write a diary entry about my failure as a nightingale, based on "The Frog and the Nightingale"?
You need to think as if you were the nightingale in order to be able to write a diary entry expressing her feelings of failure. If the entry is recording your feelings after failing as a singer, that narrows down the amount of the action that you need to include.
Your entry might start with some memories of how wonderful it had been in the old days, when you had been able to sing beautifully and how much you enjoyed the response from those listening. You might compare the "'Bravo! " "Too divine! " "Encore!'" that you used to receive with the reactions she is now receiving. Talk about the hard work you have put into following the frog's directions and practicing -
up and down the scale
For six hours, till she was shivering
and her voice was hoarse and quivering
but it has all been wasted. Your former audience no longer raves about your singing, but has grown "tired At a voice so uninspired" and left you singing to yourself. The only feedback you receive is the continued instruction from the frog to sing with more volume and passion. You might end your entry with the promise to yourself that you will try to do so tomorrow. You won't be able to record the results of that attempt, since you die while carrying out that attempt!