This weekend, I had an experience watching someone struggle with forgiveness. My brother-in-law was struggling to forgive his dad and younger brother for selling his car stereo (it was in the younger brother's car, and they didn't take it out when they sold it). They offered to pay him for it, but he wasn't willing to forgive them. It turned up into a big argument/fight (as normally does when this BIL gets upset).
In the end, one of the brothers finally put it this way - they have asked for forgiveness. It is now in your hands. It is between you and God at this point.
The Bible says this is true. Here are some verses:
Forbearing one another, and forgiving one another, if any man have a quarrel against any: even as Christ forgave you, so also do ye. Colossians 3:13
Judge not, and ye shall not be judged: condemn not, and ye shall not be condemned: forgive, and ye shall be forgiven: Luke 6:37
For if ye forgive men their trespasses, your heavenly Father will also forgive you:But if ye forgive not men their trespasses, neither will your Father forgive your trespasses. Moreover when ye fast, be not, as the hypocrites, of a sad countenance: for they disfigure their faces, that they may appear unto men to fast. Verily I say unto you, They have their reward. Matthew 6:14-16
And when ye stand praying, forgive, if ye have ought against any: that your Father also which is in heaven may forgive you your trespasses. Mark 11:25