"It was you and me, against the world."
I think, this line sums up everything in the song- she's singing about a lost love, a broken relationship, because it *was* what they had. And they no longer have it now. It's over. Evidently, it's about the end of a relationship- but it also goes much deeper than that, because she's questioning herself on whether she was the cause of it. Or whether she's just "a fool" because he promised her forever- but he couldn't do it. They all can't, in the end.
Something we can learn from here- they all don't keep their promises in the end. Be strong, be the best you can be with yourself- and you deserve so much better than a bast*d who only lies to you.
'Unbeautiful' is gutwrenching, because it's all about the past and mistakes of the past. One can't move on if one can't forget. And the truly tragic thing is that the girl always feels as if she's become "unbeautiful".