There is no limit to God's love and forgiveness
All sins, even polytheism, are forgiven with sincere repentance. Allah says: “Say: ‘O my servants who have transgressed against their souls! Despair not of Allah’s Mercy. Truly, Allah forgives all sins. Allah is Oft-forgiving, Most Merciful.” [Sûrah al-Zumar: 53]
Allah the Almighty has said: 'O son of Adam, so long as you call upon Me, and hope in 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 in the sky and were you then to ask forgiveness of Me, I shall forgive you. O son of Adam, were you to come to Me with an earthful of sins and were you then to face Me, without having associated anything with Me, I shall grant you an earthful of pardon.'"
http://fortyhadith.iiu.edu.my/hadith42.htm
"When Allah decreed the Creation He pledged Himself by writing in His book which is laid down with Him: My mercy prevails over my anger."
We can not give better advice than the Proof of Islam, Imam Ghazali (Allah be pleased with him) who said: "If you repent then break your repentance by returning to sin, then rush back to repentance. Say to yourself, 'Perhaps I will die before returning to sin this time.' Similarly, do this a third and fourth time. Just as you have made sin and returning to it a profession, make repentance and returning to it a profession. Do not be more incapable of repentance than you are of sin, nor lose hope, and do not let the Shaytan use this to prevent you from repenting. For indeed it is a sign of good. Did you not hear his [the Prophet's] (Allah bless and grant him peace) saying, 'The best of you are every one who is often tried and often penitent.' Which means they are often tried by sin and repent much and return to Allah (Transcendent and Exalted is He) with remorse and seeking forgiveness. Remember the saying of Allah (Most high): 'And he who does evil or wrongs himself, then seeks forgiveness from Allah, he will find Allah Very Forgiving, Merciful.'[4:110]" [1] And Allah knows best and He alone gives success (tawfiq).
What is sincere repentance?
Allah Most High says, "Turn towards Allah, O believers, every one of you, so that you may be successful." (24:31)
And He says, "Ask your Lord for forgiveness and then turn in repentance to Him," (11:3) and He says, "O you who believe! Turn in sincere repentance to Allah." (66:8)
The Prophet (Allah bless him & give him peace) said, “The one who repents from sin is like the one who never sinned.” [Ibn Maja (3240)] He also said, “Remorse is repentance.” [Ibn Maja (4242) and Ahmad (3387)]
Abu Hurayra (Allah be pleased with him) reported that the Messenger of Allah (Allah bless him and give him peace) said, "Allah will turn towards anyone who turns in repentance before the sun rises from the place it set." [Muslim]
Imam Barkawi, the great Ottoman Hanafi faqih, grammarian, and sufi, defined repentance (tawba) in his Tariqa al-Muhammadiyya as,
“Going back from desire to sin, with the determination of not returning to it, in exaltation of Allah and out of fear of His punishment.” [al-Bariqa fi Sharh al-Tariqa, 3.139]
The Conditions for Repentance
The conditions for repentance are well known:
1. Leaving the sin;
2. Remorse over having committed the sin;
3. Resolve never to return to the sin;
4. (If it relates to the rights of another person, then to) Return the rights or property one wrongly took. [al-Bariqa fi Sharh al-Tariqa; Riyad al-Salihin]
If these conditions are truly met, then one can expect one's sins to be forgiven. However, one has to be very careful about how sincere one is in fulfilling one's conditions. It is recommended to seek forgiveness a lot, and to repent every time the sin comes to one's mind.
If one keeps repeating the sin, then one should find the root cause(s) of the sin and eliminate them. For example, if one falls into a certain sin because of the company one keeps, then it would be necessary to either stop keeping their company, or to change the nature of one's relationship with them.
So, what is true repentance?
Ibn Hajar mentioned that Qurtubi quoted 23 different definitions of true repentance (al-tawba al-nasuh) in his tafsir. Ibn Hajar mentioned the most important of these:
1. Umar’s words (Allah be pleased with him) that it is, “To sin and then never to return to it.”
2. To hate the sin, and the seek forgiveness for it every time it occurs to one, as Hasan al-Basri (Allah have mercy on him) said.
3. Qatada’s words (Allah have mercy on him), “To be genuine and truthful in one’s repentance,” which is what Imam Bukhari chose as the definition of true repentance in his chapter heading.
4. To have sincerity in one’s repentance.
5. To be concerned about one’s repentance not being accepted.
6. To be such that it does not need another repentance after it.
7. To be made out of fear and hope, and be accompanied by consistency in worship.
8. Like the seventh, but with the added condition that one desert those who assisted one in sin.
9. That one’s sin be between one’s eyes. [f: That is, one does not forget it.] [Ibn Hajar, Fath al-Bari]