Lucy was happy at fairy tail, with her new family and Natsu. But it all changed when Lisanna returned.
It started with one of a few guilt members forgetting her and soon many. Even her team, her best friend’s beginned to drift away before they too forgot of her. At least some still remembered but Lucy knew it was only a matter of time before they too drifted away.
It also seems there was already a silent agreement to replace Lucy and Lisanna, they didn't even have the guts to face her and that might have saddened her more than a hurtful confrontation.
It seems like she really was a replacement but she can’t blame Lisanna she hasn't done anything but as event are unfolding Lucy is losing faith and hope for her and fairytail.
It wasn't until Lucy's disappearance on a mysterious job had they realized their mistake.
But it was too late, for her and her pure soul.