They don't let you continue if you get less than a C? CHRIST! Half my 6th form would've left at AS if that were the case!
As someone else has said, try not to worry about it too much. You already HAVE the grades, the exams are over, they're not something you can change. Don't consider other options too much until you actually have your results, and even then, think VERY hard about what you want to do. If it turns out you get less than a C, are there any other 6th forms or colleges near you that you could apply to do A2 at? I think that policy is totally bogus.
There's no way of blocking out your thoughts, but try to keep your mind on other things you may be looking forward to, or the good things you already have. At the end of the day, I know its a cliche but your grades do not determine who you are as a person. A friend of mine has already got himself a really good paying job, without going to university first. He's not even got his results yet, just goes to show that they're not EVERYTHING. Just remember things like that.
Take care, my dear. I'm sure you'll have done better than you imagine. That is often the case xx