Is a person held accountable for bad thoughts and whisperings that come to him if he does not believe in them? And will his repentance from grave sins be accepted if he repents and does good deeds?

Praise be to Allah for guiding you to repent from major sins. Allah accepts the repentance of those who regret their misdeeds and transforms their evil deeds into good ones. Repentance erases what came before it, and the one who repents from a sin is like one who never sinned. Have a good opinion of your Lord and balance between fear and hope. Be kind to your parents, and strive to find a righteous wife. If you are unable, then you must fast to curb your desires. Beware of despair and hopelessness regarding Allah's mercy, and renew your repentance whenever you sin. Do not give in to the devil's whispers and his imaginations concerning Allah and His Messenger, for your rejection of these whispers is a sign of your faith. Seek righteous companionship with whom you can maintain prayer, recite the Qur'an, and seek knowledge. Listen to Islamic tapes and read about the Prophet's biography to increase your love for the Messenger, peace and blessings be upon him.