That's an interesting perspective - I hadn't thought of Fanny in terms of teenage behaviour.

That "Oh! never, never, never!" is very untypical of Fanny though - normally her behaviour is very self-controlled even when she is inwardly hyper-emotional.

Restraint of her outward behaviour has been imposed on her by the demand for her to act in the approved ladylike manner and as a defence against Mrs Norris' persecutions. Fanny also makes earnest efforts to control her own thought processes and to think about things "as she ought".

In this instance she cannot think as she ought because her objections to Henry are based on the evidence of her own eyes and intelligent reasoning rather than just on irrational emotions.

So when Edmund tells Fanny "This is not like yourself, your rational self" it feels especially unjust to me. Her objections to Henry are totally rational. Edmund mistakes the unusual vehemence with which Fanny expresses her objection for evidence that she is not being rational, rather than evidence that she really, really means what she is saying.