(Regarding the with, in speech you may hear it omitted, but I'd always include it in writing)
I was never happy [with] the way I was treated in my previous job.
From the day you started at your previous job until you left that job, you were not happy. That is all you are saying.
I had never been happy [with] the way I had been treated in my previous job.
You were not happy until some event happened. This event could have happened while you were at your previous job or after you left your previous job - context coming before or after that sentence should answer that. The event could be "getting a new job," and many would assume that if you don't say otherwise, but the event could be other things.