Complex sentence: Pillet refused to mount an opera without a role for his mistress, and therefore Donizetti abandoned his project, Le Duc d'Albe, and instead composed another new opera, La Favorite, with the role of Leonor perfectly suited for Stoltz.
Split into simpler sentences: Pillet refused to mount an opera without a role for his mistress. Donizetti decided to abandon his original project, "Le Duc d'Albe", and instead composed "La Favorite" with the role of Leonor perfectly suited for Stoltz.