What do I say about why I left my last job? - 6 months and counting

They always ask why I left, and I freeze. Any experience at all would help. I also do not want to seem like a risk.

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 6 months ago, and it settled down once I kept a simple log of every call and form.