Hey! Let me say I COMPLETELY understand how you feel. SAC based stress had me bawling every night for a week straight at one point.
However, this does NOT have to be you!
One thing I think really helped me was having a checklist of things to do before a SAC.
Set yourself some small goals, and tick them off as you go. The important part is you have to do them and do them properly. When you get up to sac morning and you're feeling stressed, look at your checklist and remind yourself that you've done EVERYTHING you could do to prepare. If your confidence is low, I find that this really helps.
Otherwise, I know it's so cliche, but listening to music can really help, if you're someone that's easily influenced by music. If so, make yourself a playlist of songs/music that makes you feel calm. These don't necessarily have to have a cheerful theme, but something that makes you feel grounded. For example, one of the songs on my playlist is lowkey about a serial killer. That's okay!
If you've tried a lot of other methods, and you can tell your stress and anxiety is affecting your performance, I highly recommend you speak to your parents and your school counselors/an external counselor. In many cases, you can get special provisions made (something as simple as you sit your sac in a room alone to help deal with anxiety) that might help. I haven't done this yet myself, but friends say it's been a godsend for them, because they can actually think clearly.
And finally, (it's been said before) MEDITATE! And I don't mean sit cross legged and chant "Om" for an hour! Sometimes even simple mindfulness and breathing exercises make a BIG difference. (There's a lot of sciency stuff about more oxygen going to your brain, but you get the gist of it) You can even do it while listening to your happy music! I find if I have like minutes before a sac, it helps to stop cramming and just breathe for a while.
I think above all else, it's important to remember that in most cases, you've done the work you need to do and one, or even a few bad sacs, don't define your study score and atar.
Good Luck!