Complex sentence: When April rejected his advances and inadvertently embarrassed him in front of the other cadets, he turned against the Star Sheriffs and cultivated a personal grudge against Saber Rider, thinking it was April's affection for Saber Rider that made her refuse his love.
Split into simpler sentences: When April rejected his advances and inadvertently embarrassed him in front of the other cadets, he turned against the Star Sheriffs. Jesse cultivated a personal grudge against Saber Rider because he thought it was April's affection for Saber Rider that made her refuse his love.