Can one repent from grave sins, and what should a girl do if she is threatened with death and the destruction of her family by a man she had a relationship with after she declared her repentance?

Allah rejoices in the repentance of His servant if they repent remorsefully and resolve not to return to disobedience. Indeed, Allah (the Most High) said in a Hadith Qudsi: "O son of Adam, were your sins to reach the clouds of the sky and then you sought My forgiveness, I would forgive you, and I would not mind."

You must seek Allah's help and rely upon Him, and do not succumb to the threats of the criminal, for the threat of this world is not equal to the torment of Hell.

Persist in reciting the Mu'awwidhat (Surahs Al-Falaq and An-Nas) and "Qul Huwa Allahu Ahad" (Surah Al-Ikhlas) three times in the morning and three times in the evening. Also, say: "In the name of Allah, with Whose Name nothing on earth or in the heavens can cause harm, and He is the All-Hearing, the All-Knowing," three times in the morning and three times in the evening.

When fearing an oppressor, say: "O Allah, Lord of the seven heavens and Lord of the Magnificent Throne, be my protector from so-and-so, son of so-and-so, and his legions from among Your creation, lest any of them inflict harm upon me or transgress against me. Glorified is Your protection, and Exalted is Your praise, and there is no god but You."

If you fear an oppressive ruler, say three times: "Allah is the Greatest! Allah is mightier than all His creation! Allah is mightier than what I fear and beware of! I seek refuge in Allah, besides Whom there is no god, Who holds the seven heavens from falling upon the earth except by His permission, from the evil of Your servant so-and-so and his soldiers and his followers from among the jinn and mankind. O Allah, be my protector from their evil. Glorified is Your praise, and Mighty is Your protection, and Blessed is Your Name, and there is no god other than You."

Seek out times when supplications are answered, abandon your old life, become righteous, and listen frequently to purposeful religious audios.