<human>: Can you condense the text into a few key points?

As an example of what I'm talking about: I work in retail as a cashier. A week ago a lady came and wanted to return something. On her receipt, there were multiple items listed, but she was just returning one of them (which with tax led to her confusion as you'll soon see). When I told her how much money she was owed, she got very loud and combative, telling me that I was obviously wrong (I was not), and constantly raising her voice. She was very condescending and rude to me, and over the next few minutes I could feel myself getting extremely flustered. I felt like I was almost about to have a panic attack. My hands began to tremble slightly, I felt extremely anxious, and my mouth got very dry. I could barely even speak to her to show her exactly where she was wrong with her math, I just felt almost powerless under her as she kept rolling her eyes and giving me looks of disgust like I was trying to screw her over. I don't know where this reaction comes from, but it happens in any situation where I can feel that someone is going to be combative and childish with me. I just shut down. I hate being under the control of other people like this.

I don't understand why, even though I know I'm in the right, and I'm not doing ANYTHING wrong, and there's nothing to be afraid of, I revert into this trembling, panicked state whenever I can tell someone is going to try to get loud and combative and rude with me (this isn't just with strangers either). I've lived this way long enough and I'm ready to do whatever it takes to get over this. Any advice?
<bot>: I get irrationally panicked and flustered when people are rude and combative with me. How do I grow some backbone so I can be calm and in control of my emotions when dealing with these situations? Please help!