What must I do after having verbally agreed to apostatize from Islam to please a Hindu girl, even though my heart did not consent to that?

You must repent and return to Islam immediately. Hesitating in Islam or intending to apostatize in the future is considered current disbelief (kufr), just as suspending disbelief on a future matter is disbelief, and assenting to disbelief is disbelief. It is not permissible to associate with a foreign woman or to marry a non-Muslim woman until she embraces Islam. One must beware of forbidden relationships, increase in seeking forgiveness (istighfar), and perform righteous deeds.