Grades aren't the be all and end all. if worst comes to worst and a resit is necessary, it isn't the end of the world. Seriously, my grades have been, well... idiosyncratic in some areas to say the least.
As for the girl, I can't say I'm an expert, but you can't let one person rule your whole bloody life. I think that if you're going to be upset so easily you have to make yourself very cold. I've done that, I still have a lot of friends, but no one is ruling my life so to speak, so long as you can have a good laugh now and again, there's no problem to be had.