Re: How I Met Your Mother: Barney and... (Spoilers ahead...)
Nora only got together with Barney on the premise that he gives up his old ways and he's proven he can't.. he won't get a second chance and there's no one around who's closer to Barney and "free" than Robin (wouldn't make any sense to build up a new character in half a season just so he can marry her).
Nora did give Barney a second chance, she was willing to forgive him for cheating. All she wanted to know is if it "meant something", and Barney said that it was. He broke up with her.
~Tighr™: Not helping the situation since 1983