We are all sinners, but we don't always have contrition, or sorrow for our wrongdoing. We might know that what we did was wrong, but we aren't "sorry" about it.
The examples of St. Augustine and St. Monica can be inspirational for us, showing us the power of prayer as well as the grace that comes from a contrite heart.
Here is the collect prayer from the feast of St. Monica that asks God for an increase in our sorrow for the sins we commit.