bane of one’s existence, the

The agent of one’s ruin or misery; a THORN IN THE FLESH. The earliest meaning of the noun bane was “murderer” and was so used in Beowulf (c. A.D 800). A somewhat later meaning was
“poison,” which survives as part of the names of various poisonous plants, such as henbane or wolf ’s bane. The current sense, an agent of ruin, dates from the late 1500s. Today it is almost always used hyperbolically, as in “The new secretary loses all my messages; she’s become the bane of my existence.”