There is the strongest link between God's forgiveness of us and our forgiveness of others. Therefore, what does it mean when we find another's sin "unforgiveable"?
Well, what does God find unforgiveable? Blasphemy against the Holy Spirit (Mark 3:28-30) - for more thoughts from me on that, see here.
So what about sins we find unforgiveable? What's going on there? Well, in my limited experience of my own struggles and in talking to others, it's involved blasphemy against the offended person's true god. I've spoken to people who are well aware that:
1) Christ has forgiven them,
2) that Christ commands them to forgive, and that...
3) the offences against them are minor - not only relative to Christ's forgiveness but even when compared to other atrocities in the world.
Yet they say "I simply cannot forgive that." Essentially they consider the offender to have committed the unforgiveable sin. In these cases it's not that the offender has rejected Christ (the basic issue at the heart of the unforgiveable sin). But they have opposed the offended person's real god (their "functional saviour" to use a Tim Kellerism).
I might find countless offences to be "water off a ducks back" but if someone ruins my reputation, or if they harm my career or if they in any way hurt my children - that's unforgiveable. At those moments it's good to be aware that "unforgiveable" is synonymous with "sacrilegious." And it's good to identify the real god who we think is being blasphemed.
When the idol of "my reputation" or "my career" or "my family" is uncovered, it's actually a huge step forwards in forgiveness. Because now there's something very concrete for me to repent of. You see, she may have ruined my reputation. But I worshipped it. My eyes are taken off the horizontal for the moment and fixed on the vertical. I realize I'm not so much "offended party" as "offender". In the language of Matthew 18, I start to realize the vastness of the ten thousand talent debt. And the 100 denarii becomes instantly relativized - not just in theory, but hopefully as a felt reality.