nemesis

Use the word nemesis to describe someone or something that always causes you major problems, like the runner on another school's track team who, for years, has been beating your time by a fraction of a second.

The word nemesis describes a rival who just somehow seems able to get the best of you. It can be someone you compete against, someone whose skills are nearly identical to yours and yet, your nemesis always seems to finish ahead of you, get a higher grade, and generally make you feel flustered. Nemesis can also refer to something that always causes you problems, like public speaking, the nemesis of those who get tongue-tied when nervous.