Complex sentence: Fleur did not manage to rescue Gabrielle in time because she got held back by the Grindylows and was forced to return, but luckily Harry Potter had already saved her ever since that her attitude towards Harry had changed completely.
Split into simpler sentences: Fleur did not manage to rescue Gabrielle in time because she got held back by the Grindylows and was forced to return, but luckily Harry Potter had already saved her. Ever since that her attitude towards Harry had changed completely.