firstly, dump the daft bitch. and try to do it when you're sober so that you'll remember that she IS a bitch (what are you doing getting drunk during finals?)
secondly, forget the hsc. there are other ways to get into the uni course you want. don't let it get you down because if you really think about it, it's not worth all the stress and depression (am i trying to convince you or myself? heh...)
edit: scrap that, don't forget the hsc. stop drinking, get to studying, try and do as well as you can given the time you have left. but don't let the implications of this milestone get you down. once it's over, it's over. just gotta move on... the same could be said about your girlfriend.