spread oneself thin

spread oneself too thin (оverextend oneself, undertake so many enterprises (things) that you can do none of them well) —распыляться, разбрасываться; распылять силы; не сосредоточиваться на чем-л. одном; браться за все и ничего не доводить до конца; ||~ работать на износ

Example 1: "Don't spread yourself too thin or you'll get nowhere! " — Не разбрасывайся так, не распыляй силы — так ведь ничего не добьешься.

Example 2: I'm too busy these days. I'm afraid I've spread myself too thin. (NTC's)

Example 3: It's a good idea to get involved in a lot of activities, but don't spread yourself too thin. (NTC's)

[работать на износ]
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 (answers.com)