Complex sentence: The next night, as everyone learns Ben's motives for the shooting, Sean has a flashback about how when he was unable to tell April about his feelings for her, causing him to break a mirror, cutting his hand in the process.
Split into simpler sentences: The next night, as everyone learns Ben's motives for the shooting, Sean has a flashback about how when he was unable to tell April about his feelings for her. This flashback causing him to break a mirror, cutting his hand in the process.