Let me ask you a question: When someone has hurt or offended you, have you always—without exception—forgiven them? If you are honest, you'll probably have to admit that you haven't always forgiven others the way you should have.
Why? The reason is because we are all sinners and we aren't perfect. Instead of forgiving others, we allow our hearts to be filled with anger and hatred. Even when we have given our lives to Christ, we still do things we know we shouldn't do, and we still fail to forgive others the way we know we should.
In other words, don't be surprised if people fail to forgive you. God is perfect—but we aren't, and neither are other people. The problem is within our hearts; as the Bible says, "The heart is deceitful above all things and beyond cure. Who can understand it?" (Jeremiah 17:9).
That is one reason why we need Christ, for only He can change our hearts and replace our bitterness with His love. Make sure of your own commitment to Christ, and rejoice that He took away all your sins when He died on the cross for you. Then pray for those you have hurt and don't be bound by the past—even if they are. They need God's forgiveness also.