Idiom【草木皆兵 cǎo mù jiē bīng】

　　Bushes and trees look like enemies. Story: In AD 383, the king of Former Qin, Fu Jian, led a huge army to attack Eastern Jin. Unfortunately, he lost the first round of fighting, when he looked down from a city wall, he was terrified by the orderly army of the Eastern Jin. Then grasses and trees on the mountains arround seem soldiers of the enemy. As a result, when the nervous Fu Jian led his army into battle, it suffered a crushing defeat.

　　This idiom describes how one can defeat oneself by imaging  difficulties.