Repents sincerely

According to the verse of the Holy Quran and Mutawatir (widely related) traditions all the sins which man commits are pardonable if he repents sincerely. It is wrong to say that such a sin can never be pardoned. The Almighty Allah says in the Glorious Quran, 
"And He it is who accepts repentance from His servants and pardons the evil deeds and He knows what you do." 
(Surah Shura 42:25)

Allah has mentioned His names as:

1) Tawwabun (one who accepts Tawbah) 
2) Gaffarun (one who pardons)
3) Ghafoorun (one who forgives totally)
4) Ghaaferuz Zanbe (Pardoner of sins) 
5) Qaabelut Tawbah (one who accepts repentance)

Allah has given a general invitation to all the people to turn towards Him and seek forgiveness for their sins. If we study the meaning of the 54th verse of Surah Zumar we find that sinners have no cause to lose hope. This Ayat is also known as Ayat-e-Rehmat (The verse of Mercy).

"Say: O my servants! who have acted extravagantly against their own souls, do not despair of the Mercy of Allah ; Surely Allah forgives the blunders altogether; surely He is the Forgiving, the Merciful." 
(Surah Zumar 39:53).

Unlimited Grace

A few points must be noted from the above verse. 
Firstly, Allah says, 'O my servant' and not 'O sinners!' Even though he is addressing the sinners he addresses them as 'my servants'. So that the hopelessness of the servants may be transformed into hope in His Mercy.

Secondly, he says, "Who have acted extravagantly." This denotes a soft approach. Allah does not say 'O you who have acted blatantly,' So that the sinners may not lose hope of obtaining pardon.

Hopelessness is Haraam

The third point to be noted is that the Almighty Allah has told the sinners, "Do not despair of the Divine Mercy." The use of a negative term signifies the prohibition of hopelessness. Also it means that to lose hope regarding salvation is Haraam.

The fourth point is that Allah did not stop at this, He further adds, "Surely Allah forgives the faults altogether," signifying that the statement covers all the sins.

Fifth Point: The addition of the word 'altogether' at the end signifies that the statement includes all sins without any exception.

Sixth Point: The last point is that at the end of the verse, Allah repeats, "Surely He is the forgiving, the merciful," to stress upon the fact that Allah desires and wishes to forgive those who repent.

Repentance of the Killer of a Prophet is also Acceptable

Jabir Ibn Abdullah Ansari (r.a.) relates that a woman came to the Holy Prophet (s.a.w.s.) and asked, "If a woman kills her child can she seek pardon?" The Holy Prophet (s.a.w.s.) replied, 
"By Allah in whose control is the life of Muhammad (s.a.w.s.) even if this woman has killed seventy Prophets and then feels remorse and repents and Allah is convinced of her sincerity and the truth of her statement, upon the condition that she does not repeat the sin, her repentance shall be accepted. And her sins would be pardoned. Surely Allah is oft forgiving and forgives, ever so beyond measure. Verily one who repents (sincerely) is; as if he has never committed that sin." 
(Liyali Akhbaar)

Despair in the Acceptance of Prayers is Also Improper

If we pray to Allah for some worldly benefits or the Hereafter and our prayer is not answered it raises two noteworthy points. Firstly, we must know that Allah has promised to accept all prayers and Allah never goes back on His word.

As it is mentioned in the Holy book, 
"And when My servants ask you concerning Me, then surely I am very near; I answer the prayer of the suppliant when he calls on Me." 
(Surah Baqarah 2:186)

At another place Allah says, 
"And your Lord says: Call upon Me, I will answer you." 
(Surah Momin 40:60)

The second point is that due to His unlimited wisdom He may delay the acceptance of some prayer. So if we do not get immediate gratification we should not lose hope of having our prayers accepted.

Prayers May Not Be Accepted Due to Sins

Sometimes the sins of a person come in the way of the acceptance of his prayers. At other times Allah may delay the acceptance due to some hidden wisdom. This would compel the person to repeat his prayers and thus he would be entitled for additional sawaab. The opportunity to invocate is itself a blessing of Allah.

Another possibility is that Allah wishes to hear the entreaties of His creature many a times before He fulfills his wishes.

Delay in the Acceptance of Prayers Causes Nearness to Allah

Sometimes there is a delay in the acceptance of prayers because, to supplicate Allah continuously is the best of worship. It is the Mercy of Allah that he may want us to continue to pray to Him. It is a means of seeking nearness to Him. For whomsoever He wishes good, He gives him the Tawfeeq of supplicating Him by delaying the acceptance of his prayers.

Allama Majlisi (r.a.) writes in the book 'Hayatul Qulub' that Imam Muhammad Baqir (a.s.) says in a reliable tradition, "Hazrat Ibrahim (a.s.) used to visit the populated cities and the desolate forests so that he could derive lessons from the creatures of Allah. One day he saw a worshipper busy in prayers. His dress was made of fur and the surroundings filled with his intonation. Hazrat Ibrahim (a.s.) was astonished at his appearance. He went towards him, sat down and waited for him to conclude his prayers. When he finished his prayers, Hazrat Ibrahim (a.s.) said, "I appreciate your way and want to befriend you. Tell me where do you stay so that I could come to meet you whenever I want." He said, "You will not be able to travel on my path." 
"Why?" 
"I walk on the surface of the water." 
Hazrat Ibrahim (a.s.) said, "The Almighty Allah who has given you the capacity to walk on water, can also enable me to do so. Come, arise, today I shall spend the night with you at your residence.

When they reached the bank of the river the man uttered "Bismillah" and stepped in the river and was soon on the other side of it. Hazrat Ibrahim (a.s.) also recited Bismillah and crossed the river. This man was astounded. Then both of them entered his house.

Ibrahim (a.s.) asked him, "Which is the most difficult day?" He replied, "The day Allah will reward and punish all His creatures according to their deeds." Ibrahim (a.s.) said, "Let us together pray that Allah may protect us from the hardships of this day."

According to another report Hazrat Ibrahim (a.s.) said, "Let us pray together for the sinful believers." The worshipper said, "I will not participate in this prayer because I have been praying for something for the past thirty years and till date it has not been fulfilled, so there is no scope in praying for something else."

Hazrat Ibrahim (a.s.) said, "O worshipper! when Allah holds a creature dear He delays the acceptance of his prayers so that he may continue to plead and supplicate Him. On the other hand when He dislikes a person He answers his prayers immediately or creates hopelessness in his heart so that he would stop praying."

Then he asked the worshipper about his prayer which has not been accepted till then. He said, "One day I was busy in my prayers when I saw a handsome boy grazing a herd of sheep and goats. I asked him whose animals were those. He said they belonged to him. Then I asked him who he was. He said that he was the son of Khalilullah (Friend of Allah) Ibrahim (a.s.) and that his name was Ismail. At that moment I prayed to Allah to let me see my 'Khalil' Ibrahim (a.s.)." Ibrahim (a.s.) said, "Now your prayer has been answered. I am that Ibrahim (a.s.)." The worshipper became extremely overjoyed and embraced Ibrahim (a.s.). He kissed his head, eyes and hands and thanked the Almighty Allah with sincerity. After this both of them together prayed for the believing men and women. 

In the tradition from Imam Reza (a.s.) where he has listed the Greater Sins, despondence is mentioned after despair (Yaas). Qunut (despondence) is described as the condition where one's heart loses hope of Allah's Mercy and that person does not even dislike the hopelessness. According to the religious scholars the difference between Yaas and Qunut is that the term 'Yaas' is used in general cases and 'Qunut' in special cases. It means that 'Yaas' describes the internal condition of one's heart. When this internal hopelessness intensifies to a degree whereby its effect becomes manifest outwardly and seemobvious to the common people, it is called 'Qunut'.

In short whoever expresses hopelessness by his speech or actions is actually a victim of 'Qunut'.