Hiya, I'm so sorry for what you're going through.
You should send one email to include the SEN, student services, head of sixth form, admin and the headmaster in the same email. This way they answer you.
Dear Head of Sixth/SEN/Student Services
and so on..
I would apply for mitigating circumstances, and have your GP write a letter for evidence.
You should be able to retake your AS Levels no matter the grade as long as it's within school. If they don't you can consider doing it externally however I appreciate it can be quite expensive as I did AS externally in maths. I really think it's worth sending this email and trying to get school to help you.
Wishing you all the best !