It might help for you to try to practice this thing called Mindfulness, which a great person, Craig Hassed, once taught me (and Thushan as well, amongst others). Mindfulness is the dispassionate appreciation of everything you do, it encourages you to focus and concentrate on the little things you do every day and basically encourages you to pay attention, now essentially the rationale is that the less time you have to think about something, the less chances you will have to stress about it.
If I had to give you some advice, I would say that first of all, don't be afraid of a hard/difficult exam, if you're confident that you know your stuff, the harder the exam, the more chances you have to differentiate yourself from the rest of the state, and the less silly little mistakes will count. So if you know your stuff but you're just making small errors, the best thing to hope for would be a hard exam, rather than an easy one. Remember that in VCE, it's not about getting the highest mark but, rather the highest rank. You might think that 98% is a good mark, but if everyone else got 99%, you're still at the bottom of the pile. You might think that 80% isn't so good, but if everyone else got 60%, then you've gotten yourself a top class mark.
Second bit of advice I can give is to just be confident, you know what you know, you can do what you can do, whatever happens will, essentially just happen, you just have to believe in your own ability and just use the exam as a means to show everyone what you know and what you can do, you can only give as much as you have, so once you've given that, why still feel bad?