What is the reason for this panic that a 15-year-old girl feels, along with her recurring fantasies, and what is its treatment?

What is afflicting you may be due to shortcomings in fulfilling God's commands or violating His prohibitions, and your heart not being attached to His love and remembrance. Therefore, we advise you to fear God, observe the obligatory duties, avoid prohibitions, and draw closer to God through supererogatory acts. We also advise you not to sit alone and to occupy your time with what benefits you in your religion and your worldly life, such as reciting the Holy Quran and reflecting on its meanings, reading beneficial books, maintaining regular remembrance of God, associating with righteous people, and attending gatherings of knowledge.