Complex sentence: From childhood, Diaghilev had been passionately interested in music, though early hopes to become a composer were dashed when Nikolai Rimsky-Korsakov told him he had no talent.
Split into simpler sentences: From childhood, Diaghilev had been passionately interested in music. However, his hopes to be a composer were dashed when Nikolai Rimsky-Korsakov told him he had no talent.