Does someone have to repent of all their sins before baptism or can they repent of the ones they struggle with most?
If you were poisoned and you were going to die and someone offered you the cure to save your life would you take it? There is a catch with the cure, you must drink every drop out of the bottle otherwise you will die. Most people would choose to drink the whole bottle and scrape it clean, but when it comes to people's sin and being cured by God they tend to only want to drink partial of the cure thinking they can hold on to their poison (sin) and still be saved.
There was a rich young man who had this problem: "Now behold, one came and said to Him, "Good Teacher, what good thing shall I do that I may have eternal life?" So He said to him, "Why do you call Me good? No one is good but One, that is, God. But if you want to enter into life, keep the commandments." He said to Him, "Which ones?" Jesus said, "'You shall not murder,' 'You shall not commit adultery,' 'You shall not steal,' 'You shall not bear false witness,' 'Honor your father and your mother,' and, 'You shall love your neighbor as yourself.'" The young man said to Him, "All these things I have kept from my youth. What do I still lack?" Jesus said to him, "If you want to be perfect, go, sell what you have and give to the poor, and you will have treasure in heaven; and come, follow Me." But when the young man heard that saying, he went away sorrowful, for he had great possessions." (Matthew 19:16-22). Sadly, even though he was following God in most parts of his life, he was holding on to just one sin that cost him everything.
Before we are baptized we must choose to turn from (repent) of all our sins that we know of (Acts 17:30). If we do not, then we are choosing to serve the Devil over God, "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).