My dearest fanny, i am writing what will not be of the smallest use to you. i am feeling differently every moment, & shall not be able to suggest a single thing that can assist your mind.—i could lament in one sentence & laugh in the next, but as to opinion or counsel i am sure none will [be] extracted worth having from this letter.
in this passage from “on making an agreeable marriage,” jane austen worries that

fanny will be insulted that austen is laughing at her

fanny will ignore her good advice

she herself feels differently with each passing moment

she has good advice to give fanny