Learn English – word that describes a person who constantly underestimates situations

Er... perhaps a gossip? You may also see gossipmonger and gossiper, but in US English just plain old gossip is the standard noun.

You could perhaps try touchy (meaning 1) (American Heritage Dictionary via the Free Dictionary):-

Tending to take offense with slight cause; oversensitive.

or perhaps hysterical (meaning 2) (Random House Dictionary via Dictionary.com):-

uncontrollably emotional.

or possibly histrionic (meaning 2) (Random House Dictionary via Dictionary.com):-

deliberately affected or self-consciously emotional; overly dramatic, in behavior or speech.

depending on the reason for the overreaction or the exact shade of meaning you are attempting to convey.