interview in a ' clinical setting' terrified

hello, can anyone please help me?
I have had my appt through and I have to travel to a centre and have my interview in a ' clinical setting '. This just fills me with terror and its not even for another 3 weeks. I don't think I can do it.
I suffer with Borderline Personality Disorder, severe depression, severe OCD and severe social anxiety. I put all of this and supplied evidence with my application but they still expect me to travel to their ' clinical setting '. I have my normal doctors appt 2 days before this appt and was wondering if it might be worth me asking him to do me a letter or if he won't do that to give me some sedatives or something. Has anyone else had to do anything similar. I have spent the morning crying and I know I will not be able to cope in the coming weeks....can anyone please help or advise me please???