To conceal evil, insult to another person is a great sin. It suffices to recall the words from the Bible, where it is said that God will forgive us our sins only if we also let offense to other people.
This feeling is terrible because it harms not only the person whom we can not forgive, but also ourselves. It leaves a deep wound behind itself.
When a person is gnawing insult, he should remember how imperfect he is, how many sins he committed. And then pray. The stronger the idea that it is impossible to forgive, the more zealously one must pray.
If a person feels that it is too difficult for him to forgive an offender, you need to imagine that this is a test that was prepared by God for him. In fact, so it is. It is necessary to pass this test with dignity. As soon as it is left behind, there will be a feeling of lightness, purity and pacification.