Forgiveness is very important. Jesus said "Do not judge lest you be judged. For in the way you judge, you will be judged; and by your standard of measure, it will be measured to you." (Matthew 7:1) We are always to forgive, as Jesus tells us to "Love your enemies, pray for those who persecute you." (Matthew 5:44)

Jesus gave a parable to illustrate the importance of forgiving in Matthew 18:21-35. There was a slave who owed his master more than he would ever be able to pay. His master forgave his debt. However, this slave was owed a small amount by another slave, whom he did not forgive. Instead, he had him thrown into prison. When the master learned the second slave was imprisoned for a small debt, he ordered the first slave jailed until the original huge debt was repaid. 

The lesson? Our sins are innumerable, even huge, and we can never pay for our sins.  Another person’s sin against us is small compared to our huge debt to God. Jesus paid our huge debt with His life, and we are forgiven. Therefore, we should forgive the minor debts of sin against us. 

There is a second issue in this question. How can one be accepted into heaven? If a person sins by not forgiving, would that keep him from being accepted by God?

The only ones who are accepted into heaven are those who accept Jesus' sacrifice (Acts 4:12), are called (Hebrews 3:1), deny themselves by taking up their cross daily (Luke 9:23), and are faithful unto death (Revelation 2:10).

When a Christian is wounded and is struggling to forgive another, he should confess his problem to God and ask for help to forgive. Forgiveness is not a mushy feeling – it is a decision to only do good to another. It is a decision to not exact vengeance. Is this not what Jesus did? When Jesus was shamed, beaten, and crucified, He did not ask God to torment His abusers. Instead He died for His abusers. Heaven is the reward for Christians who have followed Jesus’ example and who have developed Christlike characters.