Let's start with the opposite. According to Scripture, it is possible to possess a "bitter root" - a heart in which bitterness of some kind or another has taken hold.
Just as a plant's roots take hold in soil - bindweed, the gardeners curse, for example - so some past bitterness can infect the human heart - and be the cause of much trouble..........
"See to it that no-one misses the grace of God and that no bitter root grows up to cause trouble and defile many." (Hebrews 12:15)
That bitterness may be against God - perhaps we feel he let us down in the past - or it may be against some human being. Whichever, this root has never been dealt with, never purged, never dug up and cast into the fire, this offense never forgiven.
And the result is that it causes trouble and spreads to many other people, "defiles many."
An unforgiving heart not only affects the person, but spreads its defilement wherever it goes. By gossip or slander against God or man, the embittered soul pours out their sour story of woe to all who will listen.
What is the cure to an embittered heart?
The cure to a bitter heart
The Scriptures provide us with many resources to help us cultivate a forgiving heart. In the first place, we must tell God that we forgive those (which may include God!) who we feel have sinned against us in our daily prayers (Matthew 6).
Secondly, we must realise the mountain of our own sin, and how the Lord has forgiven us billions and by contrast contemplate the pennies others have sinned against us by comparison and extend forgiveness for those tiny sins (Matthew 18).
Thirdly, we decide to leave all revenge to God, and free ourselves from the awful burden of being the Judge (Romans 12).
Fourthly, we remember that one of the very properties of divine love is that "it keeps no record of wrongs" (1 Corinthians 13). God himself has put away our sins as far as the east is from the west (Psalm 103) - an astounding feat for Holy Omniscience.
The Joy of a Forgiving Heart
Nothing pollutes a heart more than unforgiveness - in essence it is the polar opposite to the Gospel. And few things bring joy to a heart more than forgiveness and forgiving-ness.
Blessed are those
whose transgressions are forgiven,
whose sins are covered.
Blessed is the onewhose sin the Lord will never count against them.