don't beat your brains out

Don't waste too much of your energy or effort on some task, especially that which is or may prove to be ultimately futile or unproductive. Don't beat your brains out trying to please these people—they'll never appreciate it. It would be great if you could get the report finished by Friday, but don't beat your brains out over it. That professor is, like, the hardest grader ever, so don't beat your brains out trying to get an A from him.