How can one cure a hard heart?

Keep seeking forgiveness of Allah and ask for His help in purifying your self.

A good dua is: Bismillahir Rahmanir Raheem, Yaa Hayyu Yaa Qayyum, Yaa Muhyil Quloob, As aluka an tuhyil qalbee, Allahumma salli ‘ala muhammadin wa aale muhammad.

In the name of Allah the Gracious the Merciful O’ the Ever Living the Self Subsistent, O who gives life to the hearts, I ask you to enliven my heart, O Allah bless Muhammad and the family of Muhammad.