If it's b.s., any kind of sentence, classic or not, won't work. It'll just add salt to the wound. Even if you really mean "I don't deserve you", it's still going to hurt her. Anyways, that sentence just makes me feel like I should keep trying. ( and it sounds just as bad as "It's not you, It's me.")There's no way to avoid hurting her.
It quite obvious that your not breaking up with her because you don't deserve her and if I can see it then she will to. Just be honest with her, even if it means hurting her feelings. She will respect you for it in the long run and you may have a chance at being friends somewhere down the line.
A sure fire way to not be friends with her is to pull out every cliche line in the book.