Think about this carefully. It is the route to national and individual healing.
If we are a wealthy banker and are angry and unforgiving of those who voted for Brexit, then this prayer gives the Brexit voter permission to be angry and unforgiving of us, as we maintain the system of oppression of the poor. The country tears in two, hatred spreads and we are all worse off.
If we are poor and homeless and are unforgiving of the uncaring nature of those who are financially safe, then that gives those in power no incentive to change the situation. And we can expect to be punished for any bad behaviour on our part.
If we are unforgiving of less than perfect service from (say) hospital A&E, teachers, or waiters then we can expect no forgiveness when we fall short of perfection in our work.
If we cannot forgive our friends or family when they say or do something that upsets us, then we must be perfect in our behaviour towards them.
The prayer asks that we are forgiven in the same way that we forgive others. If we cannot forgive others, we must maintain the same demand for perfection in ourselves; a demand that we will never be able to live up to. If we want to free ourselves from feelings of guilt or inadequacy, we must choose to forgive others first for their shortcomings, and then we can know then that we are forgiven.