Granite - even the little bit I know about your mother makes me hate her.
But
My own mother had some issues with me, and I don't like it either when anyone else tries to say she was not a good mother. I was a crap daughter and nothing like the sort of thing she expected when she had one. I was the wrong sort of daughter for her. But I don't like anyone else to tell me she was bad at it.

I think the therapist does not want to collude with your idea that the problem was you and not your mother. I think it may come to talking about why/how it makes things hard on you when she does - I don't think it will make the therapist change her mind about your mother, but talking it through with her may help you.