Isabella thinks about James in regret
She thinks she's captured Captain Tilney when she dismisses her current fiancรฉ, James Morland. In the 2007 adaptation they show her faith in Tilney was due to them having a physical relationship, a faith which she quickly looses as soon as the deed is done. I like to think the last frame of this video is her thinking of James in regret.
Isabella asks Catherine, her friend and James' sister, to intervene on her behalf, completely unaware that Catherine already knows she's jilted James.
"I am quite uneasy about your dear brother, not having heard from him since he went to Oxford; and am fearful of some misunderstanding. Your kind offices will set all right: he is the only man I ever did or could love, and I trust you will convince him of it."
- Jane Austen, Northanger Abbey, C27
It's likely that Isabella, whether she slept with the Captain or not, will long think about James Morland and what she lost.
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Video from 2007 Northanger Abbey
Music from I Bet You Think About Me by Taylor Swift