Complex sentence: Charlotte would become an important force in his life, handling many day-to-day affairs and mundane details that Martinů had trouble with; though culturally, the two were quite different, a fact that would cause problems in their marriage over the years.
Split into simpler sentences: Charlotte would become an important force in his life, handling many day-to-day affairs and mundane details that Martinů found trying. Culturally, however, the two were quite different, a fact that would cause problems in their marriage over the years.