Question for beginners: Is there a safe word or rule to follow when we have a scene that is too extreme for us to handle?

My partner has expressed to me that she has a fear of being extreme, and I think that is understandable. I am not saying it is the same as a safe word, but my concern is that she may be overly vulnerable and that our relationship is being damaged. She has expressed that she does not enjoy being extreme with me and that she would be very happy with me being in subspace while she is being extreme and that she is not sexually fulfilled in the bedroom while she is being extreme. She would prefer if I was more dominant than what she would like, and would prefer to be submissive. I have also been told that she has a fear of hurting herself during sex, but has not actually stated that specifically.