There’s a few ways that I like to unpack this. First off it’s not one and done. Accepting Jesus into your heart does not mean you can never sin again. While Jesus is in your heart, you can’t sin. When he’s not in your heart, that’s when man sins.
This is seemingly contradicted in the next clause, “whosoever sinneth hath not seen him, neither known him.” But I read it a bit differently than you might. Jesus takes away your sins. So if you happen to sin, then at that moment you don’t know Jesus and have never seen him.
The ‘never seen him’ part is somewhat metaphorical. You haven’t seen that aspect of Jesus yet. Jesus is infinitely forgiving, but you have to seek out His forgiveness. So if you murdered, and you’ve never murdered before, then though you might know the aspect of Jesus that forgave you for stealing, now you need to get to know that Jesus will also forgive you for murder too.
All you have to do is return to Jesus and walk with Him again, so He can lighten your burden again. When Jesus forgives you He takes away your ability to sin that way again, but He doesn’t turn you into a carbon copy of Jesus. You can turn away and sin again. In fact, you can’t help but do it again. For man is imperfect, and bound to turn away from God.
The answer in all cases is the same, turn back to Jesus.
Let me be clear that turning to Jesus and allowing him to forgive you is not a simple or easy thing to do. It’s a Christian trope to convert on your deathbed, but realistically speaking that’s the only time many people are motivated to do it. Letting Jesus into your heart regularly like this takes practice and dedication. The reward is, you never sin.
As for the definition of sin, sinning is how we describe acts that turn you away from God. Anything that increases the amount of suffering in the world, is sin. Sin isn’t crime, stealing bread to feed your family isn’t sin, though it might be unlawful. Crime turns yourself against society, sin turns you against God.
It’s not so much that God needs you to not sin, it’s you that needs to not sin, because you need God more than God needs you.