We should be just the opposite of what you're suggesting.

We should be self-forgetful. Not counting things, real or imagined wrongs.

We should learn how to "shrug off" slights, ever more quickly and quietly.

We should be so focused on serving God and loving God in costly ways that we have no time to remember personal slights from 5 minutes ago, let alone 50 years ago!

More sporting spirit.
More "not a worry..it was nothing".

If non-Christians conclude that Christians harbor grudges from 50 years ago (over incredibly small matters) what are they to think of Christianity? Of Jesus? Of the Gospels? Of God?

They will conclude: Why be Christian if this guy is an example...a person who nurses resentments over 50 years and wallows in self-pity.

This is precisely the opposite of what Pope Francis is encouraging. This is not an "attractive" Christianity"!

This sort of spirit breeds an ever smaller horizon of life, staleness, more self-focus, cynicism, complaint, bitterness, pessimism.