What to say at an interview when you left on bad terms?

[Nikia]

I have been looking for a job for the past 2 months. I have had multiple interviews but have had no offers. I think that a big part of my lack of success is that I don't have a good response to the question of why I left my previous job.
I resigned but I didn't have much of a choice. Not to go into too much detail, there were many changes going on in the company, a new boss, and demands that I felt were impossible being placed on me. I went from being a stellar employee to being a bad employee and couldn't seem to do anything about it or get any help on what I should be specifically doing differently. Anything that I can say that would be remotely true either sounds really bad on my end or sounds like I'd be trashing my former employer. The answer that I've given so far about there being a lot of changes and stress seems to be the best that I've said so far but it still makes me sound like I can't accept change or stress, which has probably prevented me from getting the jobs.