Thank you all for your answers. I didn't expect to get as much support for such a heavy case as mine. I might reply something else about what you wrote, but for now I'll leave it like that and I'll just think about it.
But here's another reasoning that I have : what tells me that I won't fall away again ? Because I did it once and I learned that I absolutely can't trust myself. And what if I'm not an elect ? Because if there's someone who absolutely doesn't deserve His mercy, it's me. I grew up in a loving christian family. From my birth to today, God gave me absolutely EVERYTHING that I needed to lead an abundant and godly life. I've had all the light that I needed. Even in my darkest time when I totally abandoned Him, when I was bathed deep in sins, God put christian people in my life to bring me back to Him. And still I ignored it.
What did I do with all those blessings that he gave me throughout my life ? Absolutely nothing. I've never really been grateful to Him, I've never really acknowledged the fortune that I had. I've never really had this desire to know God, or almost never. Instead, I've almost always ignored Him, always been attracted to sins. I had everything I needed to become a Godly man. But I chose sin. Why ? I have absolutely no idea. And by chosing sin, I became a very wicked person inside. I won't detail all the bads of me because it would be too long, but my biggest problem is my pride. And this pride led me to do very bad things, to have lusts. Why did I become this proud person ? I have absolutely no idea. I wish I had an explanation, but I don't.
I am a proud person and a vile sinner, and I have no excuse for that. It's not as if I had a hard childhood like, or went through a hard trial or something. No ! I'm just naturally wicked, twisted inside.
So why would God have mercy on me ? Why would I be part of the elect ? 31 years old, always known about God, always been blessed, but almost always ignored Him. And I would be an elect ?
Those who are born again cannot fall away again. God's seed will remain in you that you cannot sin anymore and the wicked one touches you not. All your trials is for your purification and you are not given a trial above what you can bear but there is always provided an escape.
1 John 3:9 KJV
Whosoever is born of God doth not commit sin; for his seed remaineth in him: and he cannot sin, because he is born of God.
1 Corinthians 10:13 KJV
There hath no temptation taken you but such as is common to man: but God is faithful, who will not suffer you to be tempted above that ye are able; but will with the temptation also make a way to escape, that ye may be able to bear it.
That means, once you are born again, you are secured, sealed, protected by God.
Psalm 37:23-24 KJV
The steps of a good man are ordered by the Lord : and he delighteth in his way.  Though he fall, he shall not be utterly cast down: for the Lord upholdeth him with his hand.
Isaiah 46:4 KJV
And even to your old age I am he; and even to hoar hairs will I carry you : I have made, and I will bear; even I will carry, and will deliver you.
"So why would God have mercy on me ?"
Even Pharoah was shown mercy by God when he showed him all his wonders in Egypt. But he refused to humble his heart that's why his heart was hardened everytime he was shown mercy.
Exodus 10:3 KJV
And Moses and Aaron came in unto Pharaoh, and said unto him, Thus saith the Lord God of the Hebrews, How long wilt thou refuse to humble thyself before me? let my people go, that they may serve me.
You have to know that sinning over and over again is an indicator that you have not yet been born again.
A born again person is a new creature who does not struggle with sin anymore. Do you know that that's what's inside the new covenant of God?
Ezekiel 36:25-27,29 KJV
Then will I sprinkle clean water upon you, and ye shall be clean: from all your filthiness, and from all your idols, will I cleanse you.
 A new heart also will I give you, and a new spirit will I put within you: and I will take away the stony heart out of your flesh, and I will give you an heart of flesh.
 And I will put my spirit within you, and CAUSE you to walk in my statutes, and ye shall keep my judgments, and do them.
 I will also save you from all your uncleannesses: and I will call for the corn, and will increase it, and lay no famine upon you.
It is a covenant, a promise, he will "CAUSE" you to obey him, he will make you obey him, none of your efforts needed but only God's power at work here (by grace you have been saved and not by your own works)
How is it possible to ever stop from sinning? And here's God's promise and answer about your fear of returning from sinning and your old way of life.
Jeremiah 32:40 KJV
And I will make an everlasting covenant with them
, that I will not turn away from them
, to do them good; but I will put my fear in their hearts, that they shall not depart from me.
He will put His FEAR
in your heart so that you will never ever depart from him (as you fear happening to you because you know how vile you are and you don't trust yourself)
Psalm 111:10 KJV
The fear of the Lord is the beginning of wisdom:
a good understanding have all they that do his commandments : his praise endureth for ever.
Proverbs 16:6 KJV
By mercy and truth iniquity is purged: and by the fear of the Lord men depart from evil.
Now what should you do?
Hebrews 3:15 KJV
While it is said, To day if ye will hear his voice, harden not your hearts
, as in the provocation.
2 Chronicles 7:14 KJV
If my people, which are called by my name, shall humble themselves, and pray, and seek my face, and turn from their wicked ways;
then will I hear from heaven, and will forgive their sin, and will heal their land.
Humility is the opposite of pride. Humble yourself before God and ask him for mercy and to grant you true repentance, to open your eyes to what you don't see so that you might obey him. Ask him to humble you and put an end of yourself, to empty you, and then fill you with his Spirit.
Ask him to teach you how to deny yourself, to carry your cross daily, to lose your life on earth. Ask, seek, knock, and the door will be opened.
Any negative thoughts you have on God, ask him to remove it from you and to lead you to the right path.
If you want to also chat, let me know, I'd be happy to talk.