Beware! you can only change f2f appt once!

I have been in receipt of pip since 2013, awarded until March 2017. Received another "how your disability affects you" form in April and duly completed it with the help and support from this life-saving site (it was the full form, not the new AR1) I received an appointment for a face to face assessment despite a letter from my GP, clinical psychologist and a copy of a medical report that evidenced a previous face to face appointment (just over a year ago) having to be stopped due to a severe panic attack and a specific request from myself and medical staff that a home visit and/or phone consultation be provided - completely ignored!
I panicked when I got the appointment in the post and realised its for a time that my carer can't manage so I stupidly phoned Atos without properly checking with my carer and just accepted the first alternative they offered me - I was taken aback by the rudeness and curt attitude of the Atos person.
As it turns out, my carer has a very important appointment themselves at that time and can't accompany me - Atos state that the appointment can only be changed once - SO CHECK BEFORE CHANGING to avoid the added stress this is causing me before my appointment next Friday on my own. It truly beggars belief that I receive the benefit because of having overwhelming psychological distress when mixing with others and cannot undertake journeys unaccompanied but the system itself puts you through it.