There comes a moment, in the process of repentance, after you have wronged someone but before they are aware of it. Many times, in this moment, we have already come aware of our transgression and have begun feeling the guilt, pain, and remorse that signifies genuine t'shuva, but we also feel fear: the fear of having to admit our action and endure the other person's anger and pain.
In this moment the temptation is often to conceal our actions. We already feel remorse, after all. We have acknowledged our wrongdoing, and may genuinely have learned from our actions, changed our ways, and vowed - truthfully! - to never do it again.
This temptation is, however, the Yetzer Ra - the wicked inclination. To deny the other person knowledge of your transgression is itself transgression. There is a teaching in Jewish law that one can only forgive sins committed against themselves; I cannot forgive you for what you did to my neighbor. Likewise, I cannot forgive myself for what I did to you.
The pain of telling the other person is the pain of healing coming it. It may not feel like it at the time. As with many medical procedures, it may cause great harm in the process of healing a greater wound. But without it you are not forgiven. At best, you have merely gotten away with it. Covered it up, buried it, and hid the evidence like a criminal escaping the police. To escape justice is not the same as to reclaim innocence.
L'shana tovah; may you have an easy fast.