Long version
Unpardonable sin scroll to page 20-23
Short version. This was addressed to the Pharisees, who had just called Jesus a servant of Satan because he cast out a demon. This is to call what is good evil, it is to utterly reject God and go the other way. That is not to to say that they could not change and seek God out, but that at that moment they are in complete objection to God and cannot be forgiven for that. There are three types of sin. Accidental (chatta’ah), weakness (Avon), and deliberate rebellion (Pesha). This is of the later. The old testament sacrifice only covered Chatta'ah. Christs sacrifice can only cover Chatta'ah and Avon. Nothing covers deliberate rebellion as that is the essence of evil. Not some ephemeral deliberate sin mind you, deliberate rebellion against God himself as satan has done. Jesus, who calls the lawful Pharisees lawless ones, just called this group the worst type of sinner, that of direct rebellion against God just like satan.
You are going to fail and sin all of your life. This is actually where faith comes into play. Faith is trusting in Christs promise to save us. He didn't say, 'sin no more and you will be saved'. He said believe in me. We all struggle with sin, a great amount of faith means trusting in God's promise to save you even when you fail miserably. It takes greater faith for a sinner to trust in Christ than a lawful man to trust in Christ. See Luke 18:9-14
To be a Christian is not to overcome all temptation. Being a Christian means there is war in your heart, between the man and the old man. The conflict is what matters. Paul says in Romans 7:15
"For I do not understand my own actions. For I do not do what I want, but I do the very thing I hate.
16Now if I do what I do not want, I agree with the law, that it is good.
17So now it is no longer I who do it, but sin that dwells within me.
18For I know that nothing good dwells in me, that is, in my flesh. For I have the desire to do what is right, but not the ability to carry it out.
19For I do not do the good I want, but the evil I do not want is what I keep on doing.
20Now if I do what I do not want, it is no longer I who do it, but sin that dwells within me. So I find it to be a law that when I want to do right, evil lies close at hand.
22For I delight in the law of God, in my inner being,
23but I see in my members another law waging war against the law of my mind and making me captive to the law of sin that dwells in my members.
24Wretched man that I am! Who will deliver me from this body of death?
25Thanks be to God through Jesus Christ our Lord! So then,
I myself serve the law of God with my mind, but with my flesh I serve the law of sin.
Your fight is to serve the law of God in your mind and to believe and trust in His son to save you. It is up to God to slay the flesh, you cannot do it by your own power. There are few Christians in prison, and yet many are in prison when they are free. Both are being challenged for greater faith. When you fail respond in faith, not guilt, those two are complete opposites.