"For as by one man's disobedience many were made sinners, so by the obedience of one shall many be made righteous" Romans 5:19. The key word in this scripture is "Made". We don't become sinners; we are made sinners. Adam is our first father, we are all related to him, we all have his blood coursing through our veins. It is not our fault that we are sinners, its Adam's fault. We have inherited his sinful nature.
Just as we were lost by a representative (Adam) we shall also be saved by a representative (Christ). Jesus in our name and on our behalf fulfills all of the demands of God's law. Jesus in our name and on our behalf atones for our sins and the sins of the whole world, 1 John 2:2.
Why do folks like you always point to the first parts of Romans 5:18 and 19 and then fail to understand that it is the last parts of those verses that Paul is focusing on? And then even when you do discuss the last parts you fail to understand what Paul is even saying.
There is really a lot to be said about that entire passage in Romans 5. I will cut it very,, very short and simply point out a couple things. First, the "For as...."so by" construction of verse 19 (similarly for verse 18) indicates that it is the "so by" that is the main point of the sentence. Second, the comparison given there is specifically the difference in the effects upon mankind (the many) of Adam's disobedience and Jesus' obedience. It is important to understand that "the many" that are affected by Jesus' obedience are the same as "the many" who are affected by Adam's disobedience.
Then, who are "the many" affected by Adam's disobedience? All, I think, would agree that that is the whole of mankind; it is every person since Adam who has ever been born. Thus we would conclude that "the many" affected by Jesus' obedience must be the same, that is, the whole of mankind; it is every person since Adam who has ever been born. But you ask, "How can that be?" The answer is in at what point in the lives of the many is Paul talking about. Clearly, again as everyone agrees, the effect of Adam's disobedience is on the just born. Therefore, that is the same for the effect of Jesus' obedience. The entire discussion in Romans 5:12-19 concerns the condition of the new born or just born.
What Paul is telling us there is that whatever you think the effect of Adam's disobedience might have been on the new born or the just born, it was negated by Jesus' obedience. If the effect of Adam's sin was that the new borns comes into the world made sinners, then the effect of Jesus' obedience is that the new borns comes into the world made righteous. Nothing in that passage is talking about what happens after that. Neither the effects of Adam's disobedience nor the effects of Jesus' obedience upon the many after being born are discussed there. That discussion is contained in the very next chapter, chapter 6. It is chapter 6 that Paul discusses the effect of Jesus obedience on the one who sins. Paul said in chapter 5 that you might have thought that Adam's sin brought original sin, but the important thing is that Jesus' perfect sacrifice brought original grace. The spirit God forms in each of us is righteous and alive. It is only after that when one sins that the spirit becomes unrighteous and dead.