There are two reasons why we should not beg the Lord to forgive our sins.
The first is that sins are not abolished, they are just relocated within us. They are laid aside when after repentance we stop doing them and start a new life. This is because there are countless yearnings that stick to each evil in a kind of cluster; these cannot be set aside in a moment, but they can be dealt with in stages as we allow ourselves to be reformed and regenerated.
the second reason is that the Lord is mercy itself. Therefore he forgives the sins of all people. He blames no one for any sin. He says, “They do not know what they are doing” (Luke 23:34) (but this does not mean our sins are taken away altogether). To Peter, who was asking how many times he should forgive a friend who was sinning against him–whether he should give forgiveness as many as seven times–the Lord answered, “I do not say as many as seven times, but as many as seventy times seven” (Matthew 18:21-22).
from Regeneration, Page 40, 41