Does Allah Forgive All Sins?
Question

I am a Muslim woman, but the Shaytan has been controlling me for a while. I committed major sins, but now I regret it. I have turned to Allah and repented to Him, but I heard that no fasting or prayer will be of any benefit to me because I have committed the worst of major sins. Is that true? Will Allah really never accept my repentance?
Praise be to Allah.Repentance is a blessing from Allah
We praise Allah for having guided you to repent, and we ask Him to make you steadfast in adhering to His religion, and to grant you a good end. You should note that you have been greatly blessed and must give thanks to Allah. How many sinners die without having repented? How many of those who are astray die before turning back to their Lord? Undoubtedly the fact that Allah has enabled you to repent is a great event in your life, and this should be a time when you start to obey Him and strive harder in worshipping Him. 
Does Allah forgive all sins?
You should realize that what you have heard about repentance , prayer and fasting not being accepted from one who has committed major sins is false, and it is speaking about Allah without knowledge. There is a great deal of evidence in the Book of Allah and the sunnah (teaching) of His Messenger (peace and blessings of Allah be upon him) to indicate that Allah accepts His slave’s repentance from all sins, no matter how great they are, and it is not permissible for anyone to try to stop a person from repenting, no matter how great and how heinous his sins. 
Allah says (interpretation of the meaning): 
“Say: O ‘Ibadi (My slaves) who have transgressed against themselves (by committing evil deeds and sins)! Despair not of the Mercy of Allah, verily, Allah forgives all sins. Truly, He is Oft-Forgiving, Most Merciful.” [39:53] 
Allah says, stating that He forgives the greatest of sins (interpretation of the meaning): 
“And those who invoke not any other ilah (god) along with Allah, nor kill such a person as Allah has forbidden, except for just cause, nor commit illegal sexual intercourse and whoever does this shall receive the punishment.
The torment will be doubled for him on the Day of Resurrection, and he will abide therein in disgrace;
Except those who repent and believe (in Islamic Monotheism), and do righteous deeds; for those, Allah will change their sins into good deeds, and Allah is Oft-Forgiving, Most Merciful.” [25:68-70] 
Does Allah forgive shirk?
This clearly indicates that Allah forgives all sins – even shirk – and it tells us of an immense bounty, which is that bad deeds are turned to good deeds. 
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said: 
“No sin is too great for Allah to forgive for the one who repents, rather He forgives shirk and other sins for those who repent, as Allah says (interpretation of the meaning): 
“Say: O ‘Ibadi (My slaves) who have transgressed against themselves (by committing evil deeds and sins)! Despair not of the Mercy of Allah, verily, Allah forgives all sins. Truly, He is Oft-Forgiving, Most Merciful.” [39:53] 
This verse is general in meaning because it is addressed to all those who repent.” (Majmu’ al-Fatawa)
Al-Bukhari and Muslim narrated from Abu Sa’id al-Khudri (may Allah be pleased with him) that the Prophet of Allah (peace and blessings of Allah be upon him) said: “There was among the people who came before you a man who killed ninety-nine people. Then he asked about the most knowledgeable person on earth, and was directed to a hermit, so he went to him, told him that he had killed ninety-nine people, and asked if he could be forgiven. The hermit said, ‘No,’ so he killed him, thus completing one hundred. Then he asked about the most knowledgeable person on earth and was directed to a scholar. He told him that he had killed one hundred people, and asked whether he could be forgiven. The scholar said, ‘Yes, what could possibly come between you and repentance? Go to such-and-such a town, for in it there are people who worship Allah. Go and worship with them, and do not go back to your own town, for it is a bad place.” So the man set off, but when he was halfway there, the angel of death came to him, and the angels of mercy and the angels of wrath began to argue over him. The angels of mercy said: ‘He had repented and was seeking Allah.’ The angels of wrath said: ‘He never did any good thing.’ An angel in human form came to them, and they asked him to decide the matter. He said: ‘Measure the distance between the two lands (his home town and the town he was headed for), and whichever of the two he is closest to is the one to which he belongs.’ So they measured the distance, and found that he was closer to the town for which he had been headed, so the angels of mercy took him.” 
Al-Tirmidhi narrated that Anas ibn Malik (may Allah be pleased with him) said: I heard the Messenger of Allah (peace and blessings of Allah be upon him) say: “Allah, may He be blessed and exalted, said: ‘O son of Adam, so long as you call upon Me and ask of Me, I shall forgive you for what you have done, and I shall not mind. O son of Adam, were your sins to reach the clouds of the sky and were you then to ask forgiveness of Me, I would forgive you and I would not mind.’” [Sahih al-Tirmidhi] 
Ibn Majah narrated that ‘Abd-Allah ibn Mas’ud (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings of Allah be upon him) said: “The one who repents from sin is like one who did not sin.” [Sahih Ibn Majah]
These verses and hadiths (prophetic traditions) indicate that Allah forgives all sins, no matter how great or how many, for the one who repents to Him. 
So strive in worship and obedience, regret the negligence and sins of the past, and note that Allah has no need of His slaves yet despite that He rejoices in their repentance, and even turns their bad deeds into good deeds . 
We ask Allah to help you to remember Him, give thanks to Him and worship Him properly. 
And Allah knows best.