Could the unforgivable sin against the Holy Spirit be as follows?
If in confession we are to be our own accusers, it’s clear that we are not to be our own judge (paraphrasing a Latin quote: No one is to be the judge in their own matter). Then we are neither free to let ourselves off nor free to condemn ourselves.
Could that mysterious sin be robbing God of his prerogative to pardon us by instead persisting in condemning ourselves?
It would make a lot of sense.