"Truly, I say to you, all sins will be forgiven the sons of men, and whatever blasphemies they utter; but whoever blasphemes against the Holy Spirit never has forgiveness, but is guilty of an eternal sin"—for they had said, "He has an unclean spirit." Mark 3:28–30 (RSV)
'Never has forgiveness,' is a very unsettling word from the lips of Jesus. People approach it in one of two ways. It is ignored as if the words were not there or we lay awake nights. We lay awake nights because we aren't sure if we have sinned this sin.
This is where it is critical to remember that a Scripture segment should not be read out of context. Just before this passage, the Pharisees had confronted Jesus for casting out demons. People were beginning to ask if Jesus could be the Messiah. The Pharisees could not abide that thought. They believed that they had it worked out, which was not part of their plan for the Messiah's arrival. They declared the following about Jesus:
And the scribes who came down from Jerusalem said, "He is possessed by Beelzebul, and by the prince of demons he casts out the demons." Mark 3:22 (RSV)
The Pharisees could not allow themselves to believe that the wonders Jesus was doing were from God. If they acknowledged that, they would have to allow at least that Jesus could be the Messiah. They chose to call what the Father was doing through Jesus, a work of the devil. They called good, evil and evil, good.
If they, or anyone, would call Jesus a servant of the devil and persistently deny that Jesus is doing the Father's will, then they have refused the mercy of God through the Holy Spirit. They condemn themselves, denying the very power that would forgive them.
C.S. Lewis, in his book The Screwtape Letters, notes that those who are in hell are there because they have chosen to be there. They willingly chose eternity separated from the love and mercy of God so they could maintain they were right and God wrong. They cling desperately to this sin, for they cannot bear the thought that they might be the wrong one.
The best advice I have received about sinning against the Holy Spirit is simply this: If you are worried that you have sinned against the Holy Spirit, don't worry. The fact that your soul is troubled is a blessing of the Holy Spirit who has opened your eyes to see your sin. Ask the Father for His mercy and be confident that He will not go back on His promise to forgive.