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 the 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 here: https://fatherdoyle.files.wordpress.com/2010/06/scruples-and-their-treatment.pdf
It is consoling to think of the many saints who reformed their lives and went on to great holiness. Prostitutes, murderers, the financially corrupt, even Satanic priests have all reformed their lives and reached a high degree of sanctity worthy of beatification or canonisation. They did not allow their past to hold them back, but trusted in God’s infinite mercy. So, too, should we.