To make a mountain out of a molehill
Literally 'to drown in a glass of water'. Describes someone who overreacts to a minor problem and treats it as if it were catastrophic.
It's a small problem, don't make a mountain out of a molehill.
She makes a mountain out of a molehill with everything.
—I'm very distressed. —Calm down, you're making a mountain out of a molehill.