I'm really sorry you're going through this. I've had a similar experience actually. I've worked retail in a flower shop and had one particular customer come in occasionally who I couldn't deal with. I found her to be scary, rude, obnoxious, pushy, and completely unrelenting. She would complain about things, and then interrupt me, then tell why she thought I was having trouble answering her questions. She was a truly horrid person. I worked in a small shop so unfortunately I couldn't just ask someone else to deal with her because most of the time I was the only one behind cash. My coping strategy with her was to try to 'shut down' emotionally from her - not to react to her bs and to just carry out the transaction as unemotionally and quickly as possible - to become 'robotic' almost. It's easier said than done, for sure. But honestly, people like these have issues that are major - you have to know in your heart and in your head that it's really NOT YOU. It's them.
Your Lady Satan sounds particularly hellish. She asked you why you were shaking and laughed? That's ridiculous and to me, signifies an abusive/sadistic personality - she knows how to push your buttons and likes the fact that she affects you. That's probably why she keeps coming back.
The bottom line is that although SHE has the problem that is causing this situation - it is ultimately up to you how you choose to deal with your feelings towards this situation. For me it helped to know that she is the one being inappropriate - and you don't need to beat yourself up over this. It's not your fault. You are handling this to the best of your capabilities. Try not to give up.
Can you tell a supervisor that you've felt disrespected by this customer? Can you ask someone else to help her when she comes in, instead of you? I think if you tell people that she has made you feel uncomfortable and has insulted you, they might know how to better handle the situation. If you have to deal with her, I suggest you try to 'detach' emotionally and not respond to her BS at all. She will still give you attitude, but at least you will be more in control of yourself.
She doesn't realize she's having a huge negative impact on my life right now. After years of getting better, I find myself back to square one again. In the last few weeks I've fallen into a deep state of depression over this.
Don't give this situation the power to make you feel as though you have fallen back to 'square one' again. You haven't. You're just interpreting the situation as negatively as possible and feeling down about it. Try doing some thought records about this. Try to see this from a realistic perspective.
She only holds power over you if you give her that power.
Good luck! and I hope you feel better soon!