What do I say about why I left my last job?, accounts payable clerk here

They always ask why I left, and I freeze. I am in Omaha, in case local advice matters. I have been practising in the mirror. Do you mention numbers?

Thanks for writing this out. I went through something close to it last year, and it settled down once I kept a simple log of every call and form.

Say it plainly and move on to what you can do. Interviewers usually move on if you do.