Is a girl who is engaged and legally bound by contract [to her fiancé] – according to Islamic law – justified in refusing to express feelings of love and longing for her fiancé, claiming modesty and shyness, or has she erred, and what should be done in this case?

If the legal marriage contract has been concluded, there is no harm for the fiancée to speak with her fiancé using words of love and flirtation. If her abstention from such speech is due to her belief that it is impermissible শরীয়াহ (legally), then this belief is mistaken. However, if it is due to intense shyness, then there is no harm. The fiancé should be patient with her and explain to her that speaking with one's husband does not contradict modesty, and he should not let this matter be a cause for disappointment, for modesty is from faith and from the ethics of Islam.