Complex sentence: Boccaccio's change in writing style in the 1350s was due in part to meeting with Petrarch, but it was mostly due to poor health and a premature weakening of his physical strength.
Split into simpler sentences: Boccaccio's change in writing style in the 1350s was not due just to meeting with Petrarch. It was mostly due to poor health and a premature weakening of his physical strength.