Make a Mountain out of a Molehill

Meaning:
Over-reactive, Blowing an issue out of proportion.

Example:
Doing badly for one essay doesn’t mean you’re going to fail the entire subject. Don’t make a mountain out of a molehill!

Did you know?
The earliest recorded use of this idiom is in 1548 and found in Nicholas Udall’s translation of “The First Tome or Volume of the Paraphrase of Erasmus Vpon the Newe Testamente” and has been a popular phrase ever since.

Reference: http://en.wikipedia.org/wiki/Make_a_mountain_out_of_a_molehill