I left my previous job to the one I have now over 6 years ago, I was stressed and unhappy and knew I needed to get out. I worried too that interviewers would want to know why I was leaving. I even posted on here (under my old username) to ask for advice.

Actually the interview for the job I have now went like a dream, no questions about why I was leaving. Maybe I was lucky I don't know (I've had another job interview since and they also didn't ask why I wanted to leave my job). But just in case it's a good idea to have a positive response. In my case I was going to say I wanted a less office based job (which was in essence true) and a more hands on role. It's good to keep it positive and keep on focusing on the job you want next.

I'm sorry you're going through this btw, and wish you lots of luck going forward. In my experience leaving somewhere you are miserable can be the right thing for your own wellbeing.