Complex sentence: But Shemp was hesitant to rejoin the Stooges, since he was enjoying a successful solo career at the time of Curly's stroke until he realized that not reviving the Stooges would mean the end of Moe's and Larry's film careers.
Split into simpler sentences: But Shemp was hesitant to rejoin the Stooges, as he was enjoying a successful solo career at the time of Curly's stroke. He realized, however, that not reviving the Stooges would mean the end of Moe's and Larry's film careers.