Unforgiveness usually affects the one who does not forgive the most, rather than the one who is not forgiven.
If we do not forgive those who wronged us, who offended us, and/or who hurt us or made us suffer some way or another, then as any connection to those persons or the incident arises, it will keep on feeling like a knife stabbing us in the heart.
Forgiveness has to take place in our hearts first, and then it will show in our behavior towards that person. Just as we recognize if there is bitterness in a person's heart towards someone by treating that person, others can recognize it in our behavior too.
Mark 11: 25-26 When Jesus was hanging on the cross, in extreme pain, He not only forgave those who tortured and crucified Him, but He even asked His Father to forgive them as they had no idea what they were doing.
Luke 23: 24 Then Jesus said, "Father, forgive them, for they do not know what they do."
Matthew 18: 21-22 Then Peter came to Him and said, "Lord, how often shall my brother sin against me, and I forgive him? Up to seven times?" Jesus said to him, "I do not say to you, up to seven times, but up to seventy times seven." 70x7=490 Times!