To engage in so many activities that one can't perform any of them well: “Last semester, Pamela tried to play basketball, serve on the student council, and work at her father's store. She quickly discovered that she was spreading herself too thin.”
spread oneself too thin
Overextend oneself, undertake too many different enterprises. For example, Tom's exhausted; what with work, volunteer activities, and social life he's spread himself too thin. This expression alludes to smearing something (like butter on bread) in such a thin layer that it does not cover the surface. Jonathan Swift used spread thin in a positive sense, that is, something should occur less often (Polite Conversation, 1731-1738): "They [polite speeches] ought to be husbanded better, and spread much thinner."