What do I say about why I left my last job?

They always ask why I left, and I freeze. There are 3 people counting on me, so I am trying to be careful. I have been practising in the mirror. What do you say?

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 got easier once I kept a simple log of every call and form.