My therapist has not actually advised this but hints that the "cut-off" means that I still find her too powerful to handle.
I'm confused about what your therapist has actually said. Hints are too vague, and can be misinterpreted. If your T does think cutting yourself means you still find your mother too powerful to handle, her emphasis might be less on the contact and more on working on your own power. I don't know, it could be slightly or completely different. I don't advise reacting to a perceived hint, though. You could possibly be putting a lot of energy into a line of thought that isn't there.
I'd suggest clarifying what your therapist thinks, and expressing to her what you think and feel. If your therapist does think sustained contact with your mother is a good idea and you don't, I think you need to discuss that. Otherwise, I don't really understand how you can work together. To me, the question seems to be more about your relationship with your therapist than your relationship with your mother.