Suppose a young man is caught stealing cars, and the judge lets him off with the following warning: “If you steal cars again, you will go to prison.” Now let us suppose the young man continues to steal cars, stands before the judge and is sent to prison for 5 years and is then released.
Was he released because he was forgiven? Or was he released because he paid the price for his crimes?
He paid the price.
If the judge warned the young man by saying, “If you steal cars again you will not be forgiven in this life time,” would that change the length of the sentence from 5 years to life? No, such a pronouncement has nothing to do with the length of the sentence. The judge would be merely pointing out that no matter how long the man lives, he will suffer the consequences if he commits that crime again.
When Jesus told his listeners that a sin against the Holy Spirit will not be forgiven, he was not telling them how long those who sinned in that way would suffer. He was merely telling them that due to the serious nature of that particular sin, they would suffer and there was no getting out of it.*
As for what the unforgivable sin actually is; Dennis Prager gives a very interesting insight as to what that might be in the following youtube video. See here.
*I gathered this insight from Thomas Talbott’s book, The Inescapable Love of God. See also Talbott Vs. Piper (On Predestination, Reprobation, And The Love Of God)