I'm sorry if I'm not much help but I remember going through a similar thing, always obsessing over every little mistake.
What I usually did was obsess over it for a day or two (you're allowed to spend some time worrying about it) and then I thought:
Did I try my best?
Could I have done anything different in preparing? Did I work hard?
How much will the mistake/s I've made count towards my final mark? (Not much, as each SAC is only worth a small amount)
Will stressing about it change anything? (No)
After that, I try to not focus on it, because I tried my best and couldn't have done anything to change the outcome, either good or bad.
For my end of year exams, I really struggled on the Methods 2 Exam (didn't end up doing as well on it as my other assessments) and it was a source of stress and worry, because I was expecting to find it easy and it wasn't. I had 2 exams the next day, so what I did was spent an hour or two thinking about it, stressing and worrying, then I put it behind me because there's nothing I could do to change it. What I had to do was move past it, and prepare for the other 2 exams which were arguably more important (they were my literature and specialist exam).
It's easy to get sad, depressed, stressed and unmotivated but you need to put things in perspective. What's happened has happened and there's nothing that can be done to change it - you tried your best and you couldn't have done anything to do any better. The best thing to do is put it behind you and prepare for the next thing, as that's what will help you the most. Also, it's a good idea to have a mini celebration after finishing a SAC! Go out for a treat now that it's over (or at least that one is). One of my favourites was choc chip waffles with froyo.
Hope this helped a bit, good luck for all your SACs!
