Complex sentence: Shapiro was a sad-sack of a detective, who always assumed some other detective would be more skilful or more insightful than he; he always thinks that the promotions he receives are undeserved.
Split into simpler sentences: Shapiro was a sad-sack of a detective, who always assumed some other detective would be more skilful or more insightful. He always thinks that the promotions he receives are undeserved.