What does scripture say about regret and repentance?
Since this is such a popular topic, here's some additional encouragement from scripture on how to remain positive and determined on the path of Krishna consciousness even if we sometimes fall short of the highest standards of devotional service:
"Sri Krishna, the Personality of Godhead, who is the Paramatma [Supersoul] in everyone's heart and the benefactor of the truthful devotee, cleanses desire for material enjoyment from the heart of the devotee who relishes His messages, which are in themselves virtuous when properly heard and chanted."
"If one reads Bhagavad-gita very sincerely and with all seriousness, then by the grace of the Lord the reactions of his past misdeeds will not act upon him." (Gita-mahatmya 2)
". . . Simply by chanting one holy name of Hari, a sinful man can counteract the reactions to more sins than he is able to commit." (Brihad-vishnu Purana)
"If one chants the holy name of the Lord, even in a helpless condition or without desiring to do so, all the reactions of his sinful life depart, just as when a lion roars, all the small animals flee in fear." (Garuda Purana)
"By once chanting the holy name of the Lord, which consists of the two syllables ha-ri, one guarantees his path to liberation." (Skanda Purana)
". . . a devotee earnestly awaits the mercy of the Supreme Lord even while suffering the painful effects of previous sinful activities. Lord Krishna explains in the Bhagavad-gita that a devotee who fully surrenders unto Him is no longer liable to suffer the reactions of his previous karma. However, because in his mind a devotee may still maintain the remnants of his previous sinful mentality, the Lord removes the last vestiges of the enjoying spirit by giving His devotee punishments that may sometimes resemble sinful reactions.
"The purpose of the entire creation of God is to rectify the living entity's tendency to enjoy without the Lord, and therefore the particular punishment given for a sinful activity is specifically designed to curtail the mentality that produced the activity. Although a devotee has surrendered to the Lord's devotional service, until he is completely perfect in Krishna consciousness he may maintain a slight inclination to enjoy the false happiness of this world. The Lord therefore creates a particular situation to eradicate this remaining enjoying spirit. This unhappiness suffered by a sincere devotee is not technically a karmic reaction; it is rather the Lord's special mercy for inducing His devotee to completely let go of the material world and return home, back to Godhead . . ."
" . . . In full Krishna consciousness, he [the devotee] regrets his past experiences in material existence. This regret is very beneficial because it purifies the living entity of material, conditional life. He then prays to the Lord to engage in His service, and at that time, Krishna grants liberation from the clutches of maya."
"There is a system in the varnashrama institution by which before death one has to undergo the process of atonement for his sinful activities. One who is always engaged in sinful activities must utilize the process of atonement called the prayascitta. Without doing so, one surely will be transferred to hellish planets to undergo miserable lives as the result of sinful activities."
"As for atonement, if one has caught even a slight glimpse of devotional service, all needs to undergo prayaschitta, atonement, are superseded. Therefore atonement is certainly unnecessary when one has achieved spontaneous love . . ."
"He [Krishna] can turn the external energy into internal by His will. Therefore by His grace the external energy, which is employed in illusioning those living beings who want to have it, subsides by the will of the Lord in terms of repentance and penance for the conditioned soul. And the very same energy then acts to help the purified living being make progress on the path of self-realization."
". . .the devotee accepts a reversal of his position in life as a benediction by the Lord and consequently offers the Lord more obeisances and prayers, thinking that the punishment is due to his past misdeeds and that the Lord is punishing him very mildly. The punishment awarded by the state or by God for one's own faults is actually for one's benefit . . . If one accepts punishment as a reward dealt by the master, he becomes intelligent enough not to commit the same mistake again."
"It is said in the shastras that by once uttering the holy name of the Lord, the sinner gets rid of a quantity of sins that he is unable to commit. Such is the power of uttering the holy name of the Lord. There is not the least exaggeration in this statement. Actually the Lord's holy name has such powerful potency."
"It is firmly declared that the steady adherence of transcendentalists to their respective spiritual positions constitutes real piety and that sin occurs when a transcendentalist neglects his prescribed duty. One who adopts this standard of piety and sin, sincerely desiring to give up all past association with sense gratification, is able to subdue materialistic activities, which are by nature impure."
" . . . Those persons directly engaged in self-realization . . . need not give up their regular duties and perform special penances to atone for an accidental falldown. . . To purify an accidental falldown, one who is engaged in transcendental realization need not adopt any procedure beyond his own spiritual practice . . . one should not practice Krishna consciousness or self-realization superficially or casually; rather, one should sincerely and earnestly desire freedom from one's past sinful life . . . one must constantly practice Krishna consciousness. Thus, essential piety is to give up material sense gratification and engage in the loving service of the Lord. One who engages his senses, mind and intelligence twenty-four hours a day in the Lord's service is the most pious person, and the Lord personally protects such a surrendered soul."
"Having awakened faith in the narrations of My glories, being disgusted with all material activities, knowing that all sense gratification leads to misery, but still being unable to renounce all sense enjoyment, My devotee should remain happy and worship Me with great faith and conviction. Even though he is sometimes engaged in sense enjoyment, My devotee knows that all sense gratification leads to a miserable result, and he sincerely repents such activities."
"Therefore, although sincerely lamenting occasional lapses into sense enjoyment, he [a devotee] never gives up his enthusiasm to serve Lord Krishna. If a devotee becomes too morose or discouraged in devotional service, he may drift into an impersonal consciousness or give up his devotional service to the Lord. Therefore, the Lord here advises that although one should sincerely repent, he should not become chronically depressed. One should understand that because of his past sins he must occasionally suffer disturbances from the material mind and senses, but one should not therefore become a devotee of detachment, as do the speculative philosophers."
"One who has thus given up all other engagements and has taken full shelter at the lotus feet of Hari, the Supreme Personality of Godhead, is very dear to the Lord. Indeed, if such a surrendered soul accidentally commits some sinful activity, the Supreme Personality of Godhead, who is seated within everyone's heart, immediately takes away the reaction to such sin."
"Since devotional service is itself the most purifying process, a sincere devotee who has accidentally stumbled on the path should immediately resume his pure devotional service at the lotus feet of the Lord. And thus the Lord will protect him, as stated in Bhagavad-gita ( 9.30)"
". . .it may be seen that a person in Krishna consciousness commits some act which may be taken as most abominable socially or politically. But such a temporary falldown does not disqualify him. In the Srimad-Bhagavatam it is stated that if a person falls down but is wholeheartedly engaged in the transcendental service of the Supreme Lord, the Lord, being situated within his heart, purifies him and excuses him from that abomination. The material contamination is so strong that even a yogi fully engaged in the service of the Lord sometimes becomes ensnared; but Krishna consciousness is so strong that such an occasional falldown is at once rectified. Therefore the process of devotional service is always a success."
"When one is elevated to the platform of devotional service by the grace of the Lord and the spiritual master, one first regrets his past sinful activities. This helps one advance in spiritual life. . . the duty of a pure devotee is to regret his past sinful activities in illicit sex, intoxication, meat-eating and gambling. Not only should one give up his past bad habits, but he must always regret his past sinful acts. This is the standard of pure devotion."
"We must be conscious of the difference between our present and past conditions and should always be very careful not to fall from the most exalted life."