What God is concerned the most about us is our hearts. God wants our hearts to be like that of children, a heart of tenderness. Certain things present in our hearts can hinder our connection with God especially in the time of prayers. One of those things can be unforgiveness. It should not be in the nature of a believer to be unforgiving.
If there is one example that Christ set for us is to be able to forgive, no matter what. We see this when He hung on the cross even after being tortured unjustly. Forgiveness has no condition as long as God is concerned. This means that no offense, no matter how great, should not go unforgiven. The heart of man naturally is hard and may be difficult to forgive. But with the help of the Holy Spirit it is possible.
One of the major work God does on a believer is brokenness. If we are not broken to the point He desires there may it may debar us from some things. Brokenness can be quite tough. Part if it may require us letting go, maybe in the area of people who we have harboured in our hearts for long. God will require us to let off those people from our hearts.
Things like unforgiveness are considered as weights and the effects of these weights is that they slow us down. We should not let anything slow us down with our walk with God.
We should always forgive those who wronged us as christians