“When you stand praying, forgive whatever you have against anyone, so that your Father who is in the heavens may also forgive you your trespasses.”
~Mark 11:25

It can be particularly difficult to forgive others when we consider their sins to be much graver than anything we ourselves have ever done.
Jehovah himself has standards for forgiveness.
For instance, Jehovah expects wrongdoers to repent in order to receive his forgiveness. (Luke 17:3,4; Acts 8:22)
But even if we cannot know the heart condition of someone who has wronged us, forgiving them is key to achieving inner peace. (Eph. 4:31,32)
What’s more, Jesus taught that we should not judge others and that we should pray for our enemies. (Matt. 5:44; 7:1,2)
It helps to remember that the person, like us, was born imperfect. (Ro. 3:23)
The Bible encourages Christians to try to conquer evil with good, and to leave matters that are beyond us in God’s hands. (Ro. 12:17-21)
And we do well to keep a humble view of ourselves, knowing that Jehovah “has not dealt with us according to our sins.” (Ps. 103:10-12)