Anything that separates us from God is sin.
There 7 cardinal vices(sin):
1. Pride
2. Greed
3. Lust
4. Envy
5. Gluttony
6. Anger
7. Laziness

There 6 sins against the holy spirit:
1. Presumption
2. Despair
3. Resisting the known truth
4. Envy of a brother's spiritual good
5. Obstinacy in sin
6. Final impenitence

What makes these sins unforgivable?
You know, God's grace has an expiration date.
These sins, every sinner must pass through them and at the last: final impenitence. At 'final impenitence' he or she does not seem to have the grace of repentance.
Before I go further, let me explain grace.

There are two primary categories of grace:
1.Actual grace.
2. Sanctifying grace.

1.Actual grace is extrinsic to the soul, meaning that it is an impulse to do good or avoid evil that is sent by God and acts upon the soul much like a tugboat gently nudges an ocean liner to move in a certain direction. Sometimes the nudges aren't very subtle, as in the case of Saul who received a massive blast of actual grace on the road to Damascus in Acts 9:1-8. This actual grace "nudged" him to convert to the Catholic faith.

2. Sanctifying grace, on the other hand, is intrinsic to the soul, meaning that it inheres or takes up residence in the soul. When Saul surrendered his heart to Christ and entered the Church the Holy Spirit infused his soul with sanctifying grace.

Okay, now that we understand grace, let continue.

When you get to final impenitence, you have lost the grace of repentance.
It means you can no longer desire repentance.

You didn't know?
People think anytime they want to repent they will.
But it is not so.

Let continue by explain the first: presumption.

1.Presumption: You commit a sin and presume that Good will forgive you. He's all mercifully and forgiving, yes.
You say one more. One more won't kill me nah. After one more, another more, on and on and on.
CCC (catechism of the Catholic church) 2092: There are two kinds of presumption. Either man presumes upon his own capacities, (hoping to be able to save himself without help from on high), or he presumes upon God's Almighty power or His mercy (hoping to obtain his forgiveness without conversion, and glory without merit).
2.Despair: now you've realize you've failed to do the right thing and want to do the right thing and now are actually thinking it's impossible to repent. Seeing as it's difficult to keep from sinning.
The frustration of falling into sin, now leads you to despire.
CCC (catechism of the Catholic church): 2091: The first commandment is also concerned with sins against hope, namely, despair and presumption:
By despair, man ceases to hope for his personal salvation from God, for help in attaining it or for the forgiveness of his sins. Despair is contrary to God's goodness, to his justices — for the Lord is faithful to his promises — and to his mercy

3. Resisting the known truth: you found out that since you can't stop, why make yourself feel bad about yourself?
So you resist the known truth.
When someone says that dress is immoral you say, impossican't. You no long see anything bad in a something that used to be a sin to you. You see how despair has graduated you to this sin?

4. Envy of a brother's spiritual good: brother here refers to all Christians.
When you see someone doing so well in his or her spiritual life, you envy them.

5. After committing a sin for a long time, you actual have made your body accustomed to it.
You don't even think before you soon anymore. It seems your conscience is dead.
Before telling he truth, you lie.
Terrible unholy words are now your 'how far' and 'what's up'.
Amongst all this, you still want to change. You carve change. Not repentance though.
Only PRAYER can gain you the GRACE (CRAVING) for repentance.

6.Final Impenitence: this part scares even the author. The grace of God has deserted you.
It means you no longer have sanctifying grace. But just the grace of survival. You are not dead yet, but you are dead in the spirit. Which is what gives the physical, life.

CCC (catechism of the Catholic church) 1864: "whoever blasphemes against the Holy Spirit never had forgiveness, but is guilty of an eternal sin."-Mk 3:29; cf. Mt 12:32; Lk 12:10.
There are no limits to the mercy of God, but anyone who deliberately refuses to accept his mercy by repenting, rejects the forgiveness of his sins and the salvation offered by the Holy Spirit. Such hardness of heart can lead to final impenitence and eternal loss.

But there is hope: PRAYER.
“Come to me, all of you who are tired from carrying heavy loads, and I will give you rest. Take my yoke and put it on you, and learn from me, because I am gentle and humble in spirit; and you will find rest. For the yoke I will give you is easy, and the load I will put on you is light.”
Matthew 11:28-30 GNBDK
http://bible.com/431/mat.11.28-30.GNBDK

Bring nothing but your wholehearted self. Don't worry if you will not be able to finish what you start, just come. Don't worry if you are not sure you want to, just come. Don't worry if you think you cannot do away with what is keeping you from Him, but come. For in time, His grace will break those hindrances.
Benedicamus Domino!

Cc: lalasticlala, ishilove, Dominique, obinoscopy