Forgiveness is releasing or freeing people from the blockage of spiritual death that is formed when they sin:
James 1:14-15(NKJV): 14 But each one is tempted when he is drawn away by his own desires and enticed. 15 Then, when desire has conceived, it gives birth to sin; and sin, when it is full-grown, brings forth death.
Spiritual death is a separation or blockage from God. This blockage or blot or spot either stops completely or hinders the flow of the life of God into the human spirit.
It is instructive to note that the sin unto death spoken of here is the unpardonable sin that is committed by a Christian who deliberately rejects Jesus as His Lord and Saviour.
Such a Christian according to Hebrews 6.4-6 must be:
iv. Have tasted of the powers of the age to come: exercised spiritual authority over Satan and his demons through prayer and intercession.
If such a Christian falls away from the faith and calls the work of the Holy Spirit the work of evil spirits, blaspheming against the Holy Spirit (Matthew 12.24-32), then there is no forgiveness for them and you are not to pray for them. Those who fall into this terrible snare start by:
2. Not watching and praying frequently (Luke 21.36) and so walk after the sin nature in the flesh rather than the divine nature in the spirit especially covetousness (2 Peter 2.10, 14).
However, this is very rare indeed at this time. However, as iniquity abounds (Matthew 24.12) in this end-time it will be more common so Christians should refrain from the things listed above. So apart from this all other sins can be forgiven.
Now we need to forgive others and ourselves by confessing our sins so that we will be free to exercise our faith and be liberated from demonic torment and oppression.
3. Pray for them in tongues to edify or build them so they can repent of and overcome sin. (Jude 1.20-21, 1 Peter 4.7-8)
If we just mentally do this without doing it from our hearts and mouths by actually praying for the people we will not release them from the blockage or spot or blot of sin. It is like a man having the keys to open a prison door and agreeing to open the door in his mind but not actually putting the key into the lock and opening the door. His mental assent alone will not get the door open.
To forgive people from the heart you need to forgive and pray for them as shown above.