I am so sorry that happened to you. I'm not a math whiz either but something I figured out was that I was far harder on myself than anyone else ever was. Most often people are much more interested in themselves than they are in whether you are using a calculator. I know my smart phone has a calculator-can you use that? It might make it a little more comfortable for you that way.
I had a Special Needs adult at the Senior Center resale shop help me yesterday and she brought out her calculator when I gave her a $10 (the purchase was for $8.15) and I kept my mouth shut because I believe in using your resources and on the job training.
Good luck, and keep up the good work!