Your mother is in a very difficult position, albeit largely of her own making. For years enough has not been said to enable her to maintain the uneasy fiction with herself that things are "OK" so long as she doesn't really think about it.
You have now been pushed by the reality of the situation to finally stop colluding with this uncomfortable fiction (colluding with your silence) and so here uneasy fictional "OK"ness has fallen apart . She is exposed -to herself -as a lifelong denier. As you rightly anticipated, she is now thrashing around trying to patch over the yawning gap, and wants you to help her. Of course she does.
It's a horrible situation for her, and I am sure you pity her. But it is of her making, hers and SFs.
Somewhere in her subconscious she has probably known this was likely, sooner or later . But she probably pushed that right away.
Can she face the truth that she was an enabler. If she wants to keep a relationship with you, she has to face that, unless you can be guilted into helping her reconstruct the fiction . So the survival of her self image depends on you re-instating the deceit.
Obviously you can't do that , but boy will she be motivated to get you to do so.
Not easy to watch -to say the least. And you probably need to accept that she has a choice too.
She may choose to learn to live with a bit more truth, to keep the relationship with her daughter. Or she may find she cannot, that she needs to rebuild the fiction, but because you are not going to unsay your piece, she has to exclude you and your version of events, to keep her fiction just about going.
You can't blame her for trying, in a way , or at least, it isn't surprising. But I think advice to grey rock and just repeatedly close her down, is the only way to go.
You might need to tell her not to even open the topic with you, sooner or later. As a pp said, it really isn't your issue now. Its over to her.