I don't have many/any ideas for rebuilding trust and safety beyond what has been discussed here. Maybe just spending time together, not digging into anything painful? Or is there some sort of "clear and protect" ritual that you could do with her space/the relationship? (I do not hold any religious or supernatural beliefs, but still find rituals like this powerful.). Also, I think she needs to be trustworthy (is she?) in order for you to rebuild trust. Also, does she own her mistakes? Does she attempt to rectify them? Does she allow herself to be vulnerable with you, to be known by you? (These are things I require from my T in order to trust him--you may not need this, but it's worth thinking through what you do need and whether she can provide it).

My heart goes out to you at the thought of you being retraumatized. I have had similar experiences with my T. We nearly terminated twice and it was absolutely devastating, and I would have been very hurt, possibly even harmed (he and I differentiate between pain and damage this way). It would have been a major re-enactment of the original trauma. We go to therapy (at least I did) to have corrective experiences and then to have the same thing that harmed us in childhood happen all over again feels untenable. But I'm wondering if there is a third option other than "rebuilding trust and safety" and "retraumatization"?

I don't know the details of your relationship with your mother, but I'm wondering if there might be a way for you, over time, to terminate with this T in such a way that you get a "corrective experience/different ending"? It might not be the one you wanted or expected (e.g., a "natural" termination when you are sufficiently healed), but it might be empowering in a different way? Like, building enough internal strength and external supports to walk away from a relationship in which our needs are not being met because we know we deserve better can be healing in and of itself, if not the ending we wanted. Not sure if this applies to your situation or not, but wanted to put it out there.