Sayings of The Holy Fathers:

It is more serious to lose hope than to sin. The traitor Judas was a defeatist, inexperienced in spiritual warfare; as a result he was reduced to despair by the enemy's onslaught, and he went and hanged himself. Peter, on the other hand, was a firm rock: although brought down by a terrible fall, yet because of his spiritual experience in spiritual warfare he was not broken by despair, but leaping up he shed bitter tears from a contrite and humiliated heart. And as soon as our enemy saw them, he recoiled as if searing flames had burned his eyes, and he took flight howling and lamenting.

St. John of Karpathos

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Strive in all things to live according to the Divine commandments, and remember that the Lord is present and sees the disposition of your heart. While fulfilling an obedience, consider that it has been given by the Lord through a person, and that your salvation depends on your zeal in fulfilling it.

St. Ambrose of Optina
Spiritual Teaching of the Optina Elders, Orthodox Life, #5, 2004

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If I despise, hate anybody, it signifies that I unlawfully exalt,unlawfully love myself - that is, my flesh. Our heart continually, flatters us, secretly exalting ourselves and depreciating others. But we must constantly see our innumerable sins in order to judge ourselves, to weep over ourselves, as for the spiritually dead. Then we shall not have time to notice the faults of others, and to condemn our neigbors for them, or to despise them; but we shall esteem them, for we shall find that they are incomparably better than us in many things. When you see men fall into various sins against yourself, against the Lord, against their neighbors, and against themselves, do not be angry with them - for there is much anger and malice in the world without your anger - but pity them from all your soul and excuse them when they offend you, saying to yourself: "Father! forgive them," for sin perplexes them; "they know not what they do".

Saint John of Kronstadt