What should a girl do who wishes to wear the niqab out of conviction that it is obligatory, but fears encountering negative situations that would affect her sensitive psyche, especially with her mother's opposition?

If you believe the niqab is obligatory, then it is not permissible to abandon it without a valid excuse, and you are not obliged to obey your mother in preventing it. What you mentioned about veiled women being harmed in Egypt is unknown, and one should not pay attention to such anxieties. Rely upon Allah, for He is sufficient for you, as He says: "And whoever relies upon Allah - then He is sufficient for him" (At-Talaq: 3).