be a thorn in (one's) flesh

To be a constant or persistent cause of annoyance, frustration, or trouble. My professor was an environmental activist when he was younger, and apparently he's been quite a thorn in the oil companies' flesh ever since. This lawsuit is a thorn in my flesh. Let's just settle out of court. My car has become such a thorn in my flesh—today is the third day this week that it wouldn't start for me!