"I disagree."
"That may be true, but ....."
"I don't think that applies in my case."
"I prefer to look at it this way ...."

I've disagreed with my T several times when he made assumptions about how I was feeling, or what something meant to me, or what something signified in a dream, etc. I would probably not disagree with me if he is stating something about psychology or therapy, etc. He's the expert in that, but I have no hesitation in saying I think he's not getting something about me. It may turn out later that I didn't want to face something, or was hiding something, or being evasive, etc etc, but I feel free to speak up to him about how I am feeling and what I think.