When you forgive someone, you are asserting that your relationship will continue as if the person never hurt you at all. Forgiveness is the purposeful act of excusing a mistake or offense. God removes your offenses and never remembers them. God forgets what you have done, and fully restores you to Him.

I, even I, am he that blotteth out thy transgressions for mine own sake, and will not remember thy sins. —Isaiah 43:5 KJV 

Sometimes you may find it truly difficult to forgive. Even though you want to forgive, there is something in your heart that holds onto the hurt or anger. In these times, ask God to help you forgive. Tell Him that you want to forgive, but lack the wisdom to know how to do it.

If any of you lack wisdom, let him ask of God, that giveth to all men liberally, and upbraideth not; and it shall be given him. —James 1:5 KJV

If you choose not to forget what a person has done to you, you are faking forgiveness. You are retaining their sin against you in your mind and heart. You are not laying your life down and loving one another. John writes:

Beloved, let us love one another: for love is of God; and every one that loveth is born of God, and knoweth God. He that loveth not knoweth not God; for God is love. —1 John 4:7-8 KJV

Then said Jesus to them again, Peace be unto you: as my Father hath sent me, even so send I you. And when he had said this, he breathed on them, and saith unto them, Receive ye the Holy Ghost: Whose soever sins ye remit, they are remitted unto them; and whose soever sins ye retain, they are retained. —John 20:20-23 KJV

What happens when you fake forgiveness? If you hold onto bad feelings for another, you cause a rift between you and God and place yourself in a dangerous moral state.

But I say unto you, That whosoever is angry with his brother without a cause shall be in danger of the judgment: and whosoever shall say to his brother, Raca*, shall be in danger of the council: but whosoever shall say, Thou fool, shall be in danger of hell fire. Therefore if thou bring thy gift to the altar, and there rememberest that thy brother hath ought against thee; Leave there thy gift before the altar, and go thy way; first be reconciled to thy brother, and then come and offer thy gift. —Matthew 5:22-25 KJV

The word Raca is a Jewish term of contempt and scorn. The root word of Raca is ‘to spit’. If you do not forgive others, God will withhold the answers to your prayers.

Therefore I say unto you, What things soever ye desire, when ye pray, believe that ye receive them, and ye shall have them. 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. But if ye do not forgive, neither will your Father which is in heaven forgive your trespasses. —Mark 11:24-26 KJV

When you hold unforgiveness in your heart, you create a foothold for the enemy. Jesus says you must always forgive. He follows this with a parable to illustrate what happens when you experience the forgiveness of God, and then refuse to extend this forgiveness to another.

Therefore is the kingdom of heaven likened unto a certain king, which would take account of his servants. And when he had begun to reckon, one was brought unto him, which owed him ten thousand talents. But forasmuch as he had not to pay, his lord commanded him to be sold, and his wife, and children, and all that he had, and payment to be made. The servant therefore fell down, and worshipped him, saying, Lord, have patience with me, and I will pay thee all. Then the lord of that servant was moved with compassion, and loosed him, and forgave him the debt. But the same servant went out, and found one of his fellowservants, which owed him an hundred pence: and he laid hands on him, and took him by the throat, saying, Pay me that thou owest. And his fellowservant fell down at his feet, and besought him, saying, Have patience with me, and I will pay thee all. And he would not: but went and cast him into prison, till he should pay the debt. So when his fellowservants saw what was done, they were very sorry, and came and told unto their lord all that was done. Then his lord, after that he had called him, said unto him, O thou wicked servant, I forgave thee all that debt, because thou desiredst me: Shouldest not thou also have had compassion on thy fellowservant, even as I had pity on thee? And his lord was wroth, and delivered him to the tormentors, till he should pay all that was due unto him. So likewise shall my heavenly Father do also unto you, if ye from your hearts forgive not every one his brother their trespasses. —Matthew 18:23-35 KJV