Hopelessness and despair sometimes creep into a person’s consciousness because he does not sufficiently believe in the power of repentance. As the prophet taught us: As long as a person is alive, he can still return to God. There is therefore no reason for a person to despair as a result of of his sins.

“What shall a living man complain? Each man for his sins. Let us search and examine our ways” (Lamentations 3:39–40). This verse speaks about a living individual, [and tells us] that he has no reason to complain about his sins – saying that since he sinned he has no way to repair what he did and he will not be able to attain purity. For as long as he is still alive he should not give up hope, because despair is not justified unless a person died in his sinfulness. But while he is still alive, at any time or moment he is able to seek out and find a cure for his sickness, through something that applies equally to all of us. “Let us search and examine our ways, and return to the Lord” (Lamentations 3:39–40). “The gates of repentance are always open” (Devarim Rabba 2:12), and we are obligated to “lift up our heart with our hands” (see Lamentations 3:41), so that our hearts should be given and handed over to our hands to direct it to the good path, directed toward Him, may He be blessed, who dwells in heaven. (Rabbi Azarya Figo, Bina LeIttim, Derush 61)

There is no reason in the world for despair. No matter how low a person’s spiritual level sinks, there is always hope for him.

The principle is that it is forbidden for a person to make himself lose hope. Even if someone is a simple person and does not know at all how to study Torah, or if he is in a place where he is not able to study Torah, nevertheless, even in his simplicity he must strengthen himself in fear of God and simple service [of God] appropriate for his level…. Even someone who is, God forbid, at the lowest level – the All-Merciful One should save us from this – even someone who is placed in the depths of hell – the All-Merciful One should save us from this – since he holds himself where he is, he should still have hope of repenting and returning to Him, may He be blessed. (Rabbi Naĥman of Breslov, Likutei Moharan 2:78)

Further reading: Read the story of Rabbi Elazar ben Dordaya and his “last minute” repentance in A Concise Guide to the Sages, p. 464.

The nation of Israel was formed specifically after everyone had given up hope on the possibility that our patriarch Abraham and our matriarch Sarah would have a child. Then Isaac was born, and from him came the nation of Israel. This teaches us to believe that we should never give up hope.

The entire formation of the Israelite nation was after complete despair, that “Abraham and Sarah were old” (Genesis 18:11), and [in the words of Sarah herself], “Who would have said of Abraham that Sarah would nurse children, as I have born a son for his old age?” (Genesis 21:7). No one even considered believing that they would have a child. Even after the angel’s promise, the righteous Sarah, who knew and believed that God, may He be blessed, is omnipotent, still laughed within, for it was so far removed [from reality] to believe in this, knowing of Abraham’s old age…and her own old age. She also thought that if it was God’s will that He would remember them [and they would have a child], He would have remembered them earlier. For it is preferable [for God] to minimize a miracle, and He does not perform an unnecessary miracle. But, in truth, this was from God, so that the building of the nation would specifically come about after complete despair, when no person – even Sarah – believed that they would be remembered by God. For this is the total essence of an Israelite, to believe that one should not at all give up hope. God, may He be blessed, can always help, and, as the verse states: “Is any matter beyond the Lord?” (Genesis 18:14). One should not investigate why God did it that way. (Rabbi Tzadok HaKohen of Lublin, Divrei Sofrim 16)