In A Christmas Carol, what is the fair young girl in the mourning dress saying to Scrooge?

In The Christmas Carol, the fair young woman in the mourning dress is Belle, Scrooge's former fiance.

Scrooge receives a glimpse of the past when the ghost of Christmas Past leads him through many pertinent scenes during his younger years. When he sees the scene of himself with the beautiful Belle, his heart is broken. In the scene, Belle is breaking off her engagement to Scrooge. She tells him that she has been replaced in his affections by avarice and greed. She asserts that it is the prospect of gain which encapsulates his whole existence now, to the exclusion of everything else, even her place in his heart. Lamenting that he was once a different man, she notes that every single one of his nobler attributes seem to have left him since the pursuit of wealth became his main priority.

She tells Scrooge that their engagement was made at a time when both of them were poor; they were happy to have been of one mind then, planning for their future with abiding faith that their patient industry would soon bless them with brighter prospects. However, she asserts that Scrooge soon came to prioritize the pursuit of riches over their relationship. She says that this is the reason she must break off their engagement to marry.

Scrooge initially begs her to reconsider. He protests that he has never sought to end their engagement despite his seemingly changed attitudes towards life. However, Belle sadly reasons that Scrooge would never have chosen a poverty-stricken girl as his wife if had had a choice in his present state. She tells him that he made the choice to marry her when he was as poor as she was. So, she tells him that she will release him from his promise, as she doesn't want to be married to a man who may come to regret his decision to marry her.

But if you were free to-day, to-morrow, yesterday, can even I believe that you would choose a dowerless girl -- you who, in your very confidence with her, weigh everything by Gain: or, choosing her, if for a moment you were false enough to your one guiding principle to do so, do I not know that your repentance and regret would surely follow? I do; and I release you. With a full heart, for the love of him you once were."

After this scene, the ghost of Christmas Past shows Belle as a happily married woman with a family of her own. Scrooge begs the ghost to show him no more, as he cannot bear to witness the painful scenes of his past failures and thoughtless choices.