The extreme emotions experienced by Fanny are not unrealistic at all. Fanny's emotions are always disturbed and she has not been in a tranquil frame of mind most of the time, especially during her childhood. Her emotions are always on the surface, so to speak. I see Fanny as emotionally still in her adolescence. She has not "matured" into a woman yet in that respect. And until now, she hasn't been treated as one (a woman) either.

But there are a lot of hints that she is indeed, growing up. Her importance as a companion increases both at home, and outside. In befriending Mary, she has started developing relationships outside of her family circle and comfort zone.

Fanny’s consequence increased on the departure of her cousins. Becoming, as she then did, the only young woman in the drawing–room, the only occupier of that interesting division of a family in which she had hitherto held so humble a third, it was impossible for her not to be more looked at, more thought of and attended to, than she had ever been before; and “Where is Fanny?” became no uncommon question, even without her being wanted for any one’s convenience.

Not only at home did her value increase, but at the Parsonage too. In that house, which she had hardly entered twice a year since Mr. Norris’s death, she became a welcome, an invited guest, and in the gloom and dirt of a November day, most acceptable to Mary Crawford. Chap. 22

Her feelings of dread for her uncle appear to be going down as well. and she was thankful that she could now sit in the same room with her uncle, hear his voice, receive his questions, and even answer them, without such wretched feelings as she had formerly known. Chap 29

She is pressed even further by the Ball. She has to make conversation and dance with several strangers. Even though she is not completely at her ease, she still finds pleasure in hearing herself admired. . Fanny saw that she was approved; and the consciousness of looking well made her look still better.

she was happy in knowing herself admired Chap. 28