It looks like my points didn't get across. I don't recognise them, the way you reflected them back. So, here's one more attempt to state them. This time I'll use more words and eschew most flourishes.

There were three main points in that second paragraph of my 'More reversals' post.

1. Fanny's prediction (of being miserable forever) collapses quickly.

I take it that she can't both be miserable, and experience a glow of gratitude, at the same time. So her extreme prediction collapses almost immediately. She's sometimes not miserable, therefore she's not 'miserable forever'.

2. How did she achieve that? By concentrating her attention on one small aspect of her life.

When she concentrates on the relief of not being harassed by Mrs Norris as much as she feared, she succeeds (even only temporarily) in shutting out her other woes such as Sir Thomas's accusation of ingratitude.

3. Has we seen her do that before? Yes, with star-gazing.

Feeling as if there could be neither wickedness nor sorrow in the world is another way of overturning the 'miserable forever' thing.