Maybe it's a good idea to have a moment of evaluation? Just to mention where you think you're at and how you really feel this therapy is going and then she can do the same.
It could be a starting point to talk about it again, but also for you to mention that you really think it does work (despite the fact that you sometimes tell her it's not), no?

I once told my T how I felt about my sessions. I knew there wasn't a lot she could do for me. Only listening, as my problem at the time was rather medical. I could imagine how it must be frustrating for her and because I wondered whether she thought she could make a difference there, I brought it to the table. I think it was a good idea after all.

T's are also only human beings you know. Sometimes they might doubt themselves I guess. Maybe that's where her reaction comes from. Then why not giving it a try to reassure things a bit?

Just an idea... ;-)