What do I say about why I left my last job?, customer-service rep here

They always ask why I left, and I freeze. This is my first time dealing with any of this. I have been practising in the mirror. What do you say?

Thanks for writing this out. I went through something close to it last year, and it got easier 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.