Recently I read the account of St. Catherine of Genoa's conversion experience.
Up to then she was not a great sinner, but an ordinary Catholic probably like a lot of us.
One day she went to confession and suddenly she was struck by a tremendous awareness of two things: her own sinfulness, and God's overpowering love for her. She was so greatly shaken that she couldn't continue the confession and excused herself.
It took her a few days to get over what she felt from this experience. On the one hand, she had an incredible awareness of her sins. She could only say after this, "No more world, no more sin!" She saw her interior state with a special divine light, a light that God gave her. As a poor comparison, think of a room you normally go in that seems clean. Then a ray of light comes in and you can see the dust floating around in the air and the streaks on the windows, things you didn't notice before.
On the other hand, she had an absolutely overpowering experience of God's love. I think it would not be possible for us to truly grasp our sins unless God also gives us that experience of his love. Only when we know that God loves us totally can we face our own reality. Catherine said, "Oh Love! no more sin, no more sin!"
After a few days she went back to confession. We can imagine that this confession was completely different from any other one she had made until then. It marked the turning point of her life.
Her experience could be called an illumination of conscience. It's a special grace God sometimes gives to those he especially wants to call to conversion. St. Paul also experienced something like that on his way to Damascus. It shook him up too and he also spent three days recovering from it. But the root of this illumination is always love. We can't bear the sight of our sins unless God's love sustains us.