to be honest, getting overwhelmed in exams is not something you can claim extenuating circumstances for, neither is forgetting your glasses or misreading the weighting of questions... I know you're suffering from anxiety generally but you've been discharged from services so your therapist can't verify your current state of mind and it's very hard for a doctor to verify something after the fact, you should really go see them before the exam
I doubt you will get anywhere with your claim to be honest, I wouldn't feel this is a reflection on the universities attitude towards mental health as it's really just a lack of evidence on your part combined with reasoning which isn't really cause for IP