As has been said before the temptation to not forgive can be amazingly strong, even justified. Only living with a forgiving disposition keeps us in harmony with Jesus.
And He warns about the serious consequences that not forgiving brings.
He gave an example in Matthew 18:
"... if your brother sins against you, go and tell him his fault between you and him alone. If he hears you, you have gained your brother," v.15.
Later Peter asked for clarification:
He "... said, 'Lord, how often shall my brother sin against me, and I forgive him? Up to seven times?
"Jesus said to him:
'I do not say to you, up to seven times, but up to seventy times seven'," verses 21-22.
And He gave an illustration in verses 23-34:
"... the kingdom of heaven is like a certain king who wanted to settle accounts with his servants".
This is about a servant who was unable to repay an enormous debt he owed his master. So he pleaded with him.
His master, "moved with compassion, released him and forgave him the debt".
Then this same servant decided to call in a small debt owed to him by another servant who was unable to pay and who pleaded with him.
But there was no compassion or forgiveness or generosity of heart forthcoming from the servant even though he had been forgiven so much.
Perhaps he didn't realise it, but there were witnesses:
"So when his fellow servants saw what had been done, they were very grieved and came and told their master all that had been done" (v31).
But the same master who had been "moved with compassion":
"... delivered him to the torturers until he should pay all that was due to him" (v35).
Jesus ended His illustration with this warning:
"So My heavenly Father also will do to you if each of you, from his heart, does not forgive his brother his trespasses," (v35).
If a Believer willingly refuses to forgive there are serious consequences because there is only one course of action left open to our Father.