Identify the poor word choice and replace it with a word of your own. My endurance with the task was waning.

In this sentence, the poor word choice would be waning. It is not very common in this context, you could use a similar word, with the same meaning, and that "feels more comfortable" with the sentence, as it referred to a casual subject and, in my opinion, it has to be easy to read.

You could use words such as fading, reducing, vanishing, disappearing.

Hello again; )

The poor word choice is, 'waning'.

Some replacements could be, "Tiring, weary, exasperating, frustrating, troublesome,"