Can she say stop

I have to admit I am slightly worried about her ability to say stop once we have started a scene. She was initially worried that I would stop when she did not need me to I am guessing she thought I had a gentle nature.

She has said something that worries me that she is so afraid of disappointing me she is not sure she can use her safe word if she needs to. I guess that will be something that I have to find out will she let it go so far that I break her without meaning to?