You must absolutely drive away all despondency or useless pining or regrets about the past. It does not please God and it only injures your spiritual life.
COMMENT: Worrying about the past, and about our sins that we have already confessed, is one symptom of suffering from scruples. Fr Doyle had great concern for the scrupulous, so much so that he wrote a pamphlet on the “treatment” of scruples. The text can be found on the page on this site dedicated to his writings.
It is consoling to think of the many saints who reformed their lives and went on to great holiness: St Mary Magdalen who was caught in adultery; St Peter who denied Christ three times; St Paul who persecuted the Church; St Augustine who was addicted to lust; St Camillus who was an ill-tempered gambling addict and St Teresa of Avila who loved to gossip and waste time. Even a Satanic priest can convert, repent of his past life and reach heroic holiness, as the example of Blessed Bartolo Longo shows us. Dozens more can be added to the list. They did not allow their past to hold them back, but trusted in God’s infinite mercy.