How can one stop a sin that is deeply rooted in the heart and from which one thinks there is no escape as long as one lives?

The thought of being unable to rid oneself of sin is a satanic whisper. Repentance is possible, no matter how great the sin, through sincere struggle against one's self. A person should initiate a sincere repentance, moving away from the causes of sin and committing to supplicating to God. Let him recall the ugliness of sin and its evil consequences, and that it is like a delicious food containing deadly poison. Let him recall the greatness of God and His punishment, and death and what comes after it. Let him accompany righteous people who help him in obedience to God, and let him be sincere in repentance and cease the sin immediately. If he slips, let him return to repentance, for God's forgiveness awaits him as long as he sins and repents.