What do I say about why I left my last job? (Jacksonville)

I have an interview next week and I am unsure how to explain why I left my last job as an insurance claims processor. This is my first time dealing with any of this. I also do not want to seem like a risk. What do you say?

Thanks for writing this out. I went through something close to it 14 months ago, and it got easier once I kept a simple log of every call and form.