You can retake whichever individual exams/units you want, and (I think) you can retake as many times as you want.
Last year I managed to completely avoid thinking about results by travelling in Europe from a couple of weeks after my exams ended until the day results came out. That was pretty effective. Keep busy - get a job, arrange a holiday/day out with friends, find yourself a new hobby (sport? dancing? an instrument?), make a list of things to read/watch/do and then read/watch/do them, have endless quarrels with your little brother (or someone else's little brother, if you're lacking one of your own)...just don't let yourself sit there and worry.