What do I say about why I left my last job? - 3 weeks in

I have an interview next week and I am unsure how to explain why I left my last job as an office manager. I have been practising in the mirror. Do they ask follow-ups?

Practise it out loud with a friend. Mine felt natural after the third try.

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.