This has been bothering me for a while, so I am going to explain my situation. I was baptized as an infant and grew up knowing all about Jesus and what is expected of a Christian. However, as a young adult, after studying the Bible, I realized my baptism was done wrong. I remember feeling this almost urgency to do it the right way, telling myself that if the Bible says to be baptized, what was stopping me? So, I was baptized as a young adult.
I have two reasons as to why I am worrying I need to get re-baptized. First, the repentance part. It wasn't until after I was baptized that I started noticing things in my life that were really messed up, things that I never even saw as messed up before. I started to work hard on changing, and there have been major improvements, though I still slip up occasionally. My other reason has to do with doubt itself. When I got baptized, I knew, and still know, that I need Jesus because I can't have an eternal relationship with God otherwise. Over the past several months, I have been doubting, not that I am a sinner who needs Jesus, but that Jesus did what he did for me. I know he died for everyone else, but I'm just anxious a lot of the time that God just does not want me, so Jesus did not die for me. I sure want God, but I'm just scared He does not share that same desire. I don't know if Satan is causing me to feel this anxiety, or if I really do have a reason to be anxious and have to go back and get re-baptized again and start my whole faith over again.
"Therefore, putting aside all malice and all deceit and hypocrisy and envy and all slander, like newborn babies, long for the pure milk of the word, so that by it you may grow in respect to salvation, if you have tasted the kindness of the Lord" (I Peter 2:1-3).
I want you to notice that Peter is writing to Christians; yet, he is telling these new Christians they need to put away several sins that are corrupting their lives. Why? Because no one who becomes a child of God knows everything when they first start.
It is important before becoming a Christian to know that you have sinned and that you need the Lord to get out of sin. Typically a person knows of several sins that needs to be straighten out. This leads to repentance. We can take about repentance in regards to specific sins, but we can also talk about repentance in a person's general attitude toward sin. A Christian has a general attitude that sin is intolerable in his life, so when he realizes he has sinned, he does something about it. "And you know that He was manifested to take away our sins, and in Him there is no sin. Whoever abides in Him does not sin. Whoever sins has neither seen Him nor known Him. Little children, let no one deceive you. He who practices righteousness is righteous, just as He is righteous. He who sins is of the devil, for the devil has sinned from the beginning. For this purpose the Son of God was manifested, that He might destroy the works of the devil. Whoever has been born of God does not sin, for His seed remains in him; and he cannot sin, because he has been born of God. In this the children of God and the children of the devil are manifest: Whoever does not practice righteousness is not of God, nor is he who does not love his brother" (I John 3:5-10). John is not saying a Christian never sins -- that would be impossible (I John 1:8-2:1), but a Christian doesn't tolerate sin in his life.
Thus, as you grow more knowledgeable about God's law, you will find shortfalls in your life. You wouldn't be growing if this did not happen. As a Christian, that means you will make changes to get the sin out of your life. But none of this requires getting baptized again. You are still in the covenant with Christ (Colossians 2:11-14). Just as an Israelite did not get circumsized again when he fell into sin, neither does the Christian.
Baptism is not the only way sin is forgiven. Baptism is necessary to become a child of God. In that process, your sins are forgiven. But now that you are a Christian, we have the right to approach God in pray to ask for forgiveness when we sin (I John 1:9) after we repent of that sin (II Corinthians 7:10-11).
Your feelings don't define truth. The Bible says that Christ died for everyone, and that includes Hitler, Sadam Husein, and you. "For the love of Christ controls us, having concluded this, that one died for all, therefore all died; and He died for all, so that they who live might no longer live for themselves, but for Him who died and rose again on their behalf" (II Corinthians 5:14-15). Now most don't avail themselves of the offer of salvation that comes from Christ's death, but you were buried in his death and his resurrection (Romans 6:3-7). Faith is having trust in God and what He said, not in yourself. To say that God doesn't want you is to call God a liar because He said, "The Lord is not slow about His promise, as some count slowness, but is patient toward you, not wishing for any to perish but for all to come to repentance" (II Peter 3:9).
Stop focusing on your past sins. Straighten out things in your life and focus yourself on the goal. "Not that I have already obtained it or have already become perfect, but I press on so that I may lay hold of that for which also I was laid hold of by Christ Jesus. Brethren, I do not regard myself as having laid hold of it yet; but one thing I do: forgetting what lies behind and reaching forward to what lies ahead, I press on toward the goal for the prize of the upward call of God in Christ Jesus" (Philippians 3:12-14).