I went to get some food from everyone's favourite golden arches, made a mistake and accidently ordered something twice but didn't realize until I paid. I was then completely ignored when I asked to have the refund placed on my card and not in cash, then proceeded to be told I was wasting their time because they were already making the order when I asked for a refund (had to wait a few minutes to be served in person as no one was out the front) and that they don't refund on card. This was the only manager on and he didn't even acknowledge my existence until I had repeated myself like 5 times. He then just walked away leaving me completely dumbfounded. 

I couldn't really deal with much more as my anxiety was taking over and I was getting shaky so I asked if I could get a full refund, where the manager proceeded to treat me the same. 

Normally I get a bit crazy and almost become a karen due to not knowing how to act and my anxiety taking but today I 100% refrained from even raising my voice as I'm really trying to work on it. I still felt like a spectacle with people staring at the "crazy customer."

I managed to get home before completely breaking down in a full panic attack and feel like a complete failure as I can't even make a complaint without breaking down or deal with terrible customer service.
  
Tldr: went to complain about terrible customer service and ended up having a panic attack.

Any suggestions on how to deal with this in the future?