Is it ever the case that the pain people feel from another’s injustice is so deep that they should just back off and not forgive?
There is a difference between backing off for a while, refreshing, and then trying to forgive again and abandoning forgiveness altogether. Sometimes we need to take that time-out because the effort and pain are too much. If we then abandon forgiveness entirely, my worry is this: What do you then do to reduce that pain? Forgiveness is a scientifically-supported way of reducing that pain and so, if a person so chooses, going ahead once again with the forgiveness process can be healing.
For additional information, see The Four Phases of Forgiveness.