I think Elizabeth's comment about remembering the past only as it gives you pleasure is an example of her 'professing opinions that are not her own'. We know that she doesn't forget the past -- for example, she says that no one can forget Wickham's wickedness.

She may believe that it is reasonable to not be consumed by the past, but I think we can be certain that she will not forget it. She will remember it, but in a positive way, so she does not repeat the mistakes of judgment that she made in the past.