Is it obligatory for the one who repents to rectify the grave sins he committed, or is it sufficient for him to forget them and start anew, like someone who embraced Islam?

One who has disobeyed Allah must offer sincere repentance, restore rights to their owners if the sin pertained to human rights, and increase their acts of seeking forgiveness and performing good deeds, all while persistently supplicating and maintaining a good opinion of Allah regarding the acceptance of their repentance. They are not required to force themselves to forget the sin or to remember it.