What do I say about why I left my last job? - any advice?

They always ask why I left, and I freeze. I have been practising in the mirror. Do they ask follow-ups?

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

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.