Complex sentence: From childhood, Diaghilev had been passionately interested in music, but his ambition to become a composer was dashed when (in 1894) Nikolai Rimsky-Korsakov told him he had no talent.
Split into simpler sentences: From childhood, Diaghilev had been passionately interested in music. His ambition to become a composer was dashed when (in 1894) Nikolai Rimsky-Korsakov told him he had no talent.