What you are describing is called reprobate. The fear of this condition has been a thorn in the side of many Christians before you, and will most likely be so for many in the future. As stated above, the unforgivable sin of blaspheming the Holy Spirit occurs when you behave as the Pharisees in Mark 3. They attributed the work of the Holy Spirit to satanic acts and therefore prevented themselves from His work within them. This is one way to fall into reprobate, or rejection.
Another way is as written in Romans 1:
"And even as they did not like to retain God in their knowledge, God gave them over to a reprobate mind, to do those things which are not convenient; Being filled with all unrighteousness, fornication, wickedness, covetousness, maliciousness; full of envy, murder, debate, deceit, malignity; whisperers, Backbiters, haters of God, despiteful, proud, boasters, inventors of evil things, disobedient to parents, Without understanding, covenantbreakers, without natural affection, implacable, unmerciful: Who knowing the judgment of God, that they which commit such things are worthy of death, not only do the same, but have pleasure in them that do them."
If you read the verse you can see that if you are of a reprobate mind you do not care about it. You take pleasure in it. God's conviction is no longer upon you. You are generally unpleasant to the righteous, and would most likely be asked to leave church for upsetting other members. There are reprobate minds out there and they are obvious. You would know if you were one because you would be proud of it.
"Examine yourselves, whether ye be in the faith; prove your own selves. Know ye not your own selves, how that Jesus Christ is in you, except ye be reprobates?" ~ 2 Corinthians 13:5
Jesus Christ is not in you at all if you are reprobate. But guilt can cause us to think that we are. Look at what it says two verses later:
Now I pray to God that ye do no evil; not that we should appear approved, but that ye should do that which is honest, though we be as reprobates. ~2 Corinthians 13:7
You still sin. You act out. Your flesh is desperately wicked and proof that you are hopelessly in dire need of a Savior. That isn't something to feel guilty about so that your life can be consumed with despair. It is merely fact. It is a truth. Reconcile yourself with the idea that God's opinion of us is reality. He loves us anyway, for His own purpose and reasons.
Here's another few verses you should concentrate on:
"Then said Jesus unto his disciples, If any man will come after me, let him deny himself, and take up his cross, and follow me." ~Matthew 16:24
"And when he had called the people unto him with his disciples also, he said unto them, Whosoever will come after me, let him deny himself, and take up his cross, and follow me." ~Mark 8:34
"And he said to them all, If any man will come after me, let him deny himself, and take up his cross daily, and follow me." ~Luke 9:23
If you love the Lord with all your heart, all your soul, and all your mind, you will choose Him over all things including sex with your girlfriend. What you have here is a symptom of not putting God first in your life. My earnest suggestion to you is to do an inventory of all those things that take your attention away from Him. What in your life distracts you from God rather than bringing you closer to Him? What do you care about most? What makes you upset? If God doesn't find the same emotional response as you for those things, they probably need to go away. Those things are keeping you from feeling the true joy you are trying to find through fornication and other sins. My earnest advice to you is to leave your old life behind, take up your cross daily, and follow Him.