Couple of queries:-
In the sentence ' As I grew up, it was her melancholy ...' the advice is to use c'est. Why not c'était?
and,
In the sentence 'Barbara, as you used to sing (it) so perfectly, our most beautiful love story will always be you., the 'it' refers to the 'love story' which is feminine so 'comme tu le chantais' should be 'la'?
Thanks