What do I say about why I left my last job? - 18 weeks in

I have an interview next week and I am unsure how to explain why I left my last job as a software tester. It has been weighing on me all week. I do not want to blame the company. What do you say?

Thanks for writing this out. I went through something close to it 12 months ago, and it settled down once I kept a simple log of every call and form.