Great news that you want to repent! I think I understand what you’re talking about.
You’re not the first to have doubts about beginning to follow Christ. In fact, Christians can sometimes think exactly these same things: Am I just talking to myself? Do I really believe this? Am I really sorry for my sins?
Thankfully, God is greater than our doubts. The fact that we turn to him at all is only possible by his gracious action in our hearts and minds. Ephesians 2:1-10 gives a fantastic description of God’s work for us, and verse 8 teaches us that even faith is a gift from God.
In the end, we are not saved by the strength of our belief, but by the cross of Christ. We’re not saved by our own certainty about Jesus, but by God’s act of mercy in history to save us and show himself to us.
There’s a story of Jesus from the gospel of Mark that is really relevant here. A boy has an evil spirit that has made him mute for years and throws him into fits. The boy’s desperate father brings him to Jesus, having seen the disciples try and fail to heal him. From Mark 9:21:
And Jesus asked his father,"How long has this been happening to him?"
And he said,"From childhood. And it has often cast him into fire and into water, to destroy him. But if you can do anything, have compassion on us and help us." And Jesus said to him,"‘If you can’! All things are possible for one who believes."
Immediately the father of the child cried out and said,"I believe; help my unbelief!" And when Jesus saw that a crowd came running together, he rebuked the unclean spirit, saying to it,"You mute and deaf spirit, I command you, come out of him and never enter him again." And after crying out and convulsing him terribly, it came out, and the boy was like a corpse, so that most of them said,"He is dead." But Jesus took him by the hand and lifted him up, and he arose. (Mark 9:21-27)
Isn’t that incredible! The man says,"If you can do anything, have compassion"" I love Jesus’ response,"If you can?! What do you mean, If you can? Trust me!"
And the man wants to believe. He says that great line:"I believe - help my unbelief." He’s saying that he believes Jesus in some way, but that he’s shaky about it and he needs help to believe more strongly. And Jesus heals his boy, as if bringing him back from the dead.
I don’t think there’s any greater cry of dependence (that is, faith or trust), than what the man says,"I believe - help my unbelief." He has just enough to ask Jesus to fill him with faith. For the person who wants to repent and become a follower of Jesus, that little stepping off, as unsure as it may be, is the start of great growth into trust of God.
Just before the bit from Ephesians above about the work of God, the apostle Paul talks about praying a prayer for the Ephesians. His prayer is that God will give them greater insight and wisdom to know God better - to trust him better (Ephesians 1:16-21). God will work in Christians to convince them of the truth of the gospel.
In terms of advice, if I were to give you any it would be to go ahead and become a Christian by repenting and telling God you want to serve him, and then throw yourself into the Bible, to get to know God better and grow in your love and trust of him. The gospel of Mark is a great place to start.
No Christian has perfect trust in God, and there are lots of enemies of God in this world seeking to undermine our mustard-seed faith in him. But we don’t need to worry. God has saved us by his grace in Jesus Christ - we all just keep saying to God,"I believe - help my unbelief."