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.

Ref: Greater sins, Vol.1