Unrepented sin sends someone to hell. We are all sinners, we all fall. BUT if we say “Oh well I’m no good any way I will just keep on doing what I’m doing so why ask for forgiveness” THAT is unrepented sin. If someone says “OH I can’t go to confession or ask God’s forgiveness because what I did is SO bad He could never forgive me” that is unrepented sin and that is saying that God’s mercy is NOT greater than anything I did—which is NOT true. Or if we just say “so what if I sin, everybody does it, it’s part of life and we aren’t perfect anyway so I’ll just be as good a person as I can be and that’s that” nope unrepented sin.
We are ALL sinners but what saves us is that we search our conscience, admit what we did was wrong, be sorry for what we did that was wrong and ASK Our Lord’s forgiveness in Confession. We must do this often.
The only difference in a Saint and a sinner is that a Saint is a sinner who keeps trying and after each fall knows they fell, admits why they fell, and ask’s God’s forgiveness in Confession.