]Is it possible that Charlotte missed an opportunity in the past of "fixing" a man and is now sorry she was so reserved towards him?

Her voice does almost seem to be the voice of experience. She might also hold such views because of her bad situation. Knowing that the Bennet girls were in an even worse situation (Charoltte at least had brothers to take care of her), she no doubt saw it as her duty to try to help her friend into a very good marrige and would be sad to see Jane throw away an oppertunity that might never come up again.

]Someone asked earlier why she interrupted Lizzy talking to Mr. Darcy and forced her to play the piano at Lucas Lodge, ... (snip) Charlotte is a good friend to Lizzy and seems to want to put her in a good light so she can have a chance at happiness. And performing for everyone at Lucas Lodge would accomplish this.