I love Frederick's reply as well, but I have to feel for Anne:

"I am not yet so much changed," cried Anne, and stopped, fearing she hardly knew what misconstruction.

As you say, Anne is anxious to let Frederick know that she still cares for him. However, she stops suddenly as she realizes what she has said could be taken another way! Perhaps Frederick will think she is saying that she has not changed her mind about refusing him!

And those eight years and a half! Is Frederick thinking that any care she once had for him is gone?