Oh yes bloopsy, I relate. I really do.
Even with a moderately distant relationship, I still feel the effects of being overshadowed by the pretense of others. In fact, the rest of my family reinforces her truth by not disputing or correcting her, then acting as her proxy by telling me "how hurt she is that we aren't close".
They want me to pretend and be the one to make her happy, so they can stop listening to her, so they will not be victimized by her distortions as well. What they don't really get is that it won't work.
She actually uses very powerful carrots to get me to forget and pretend so she does not have to be responiible for the past (she keeps trying to get me to let her help me buy a bigger, nicer house that I do no0t truly need), all the more so as she gets older.
Your Mom probably does not want to hear you ask her why she failed to protect you. The problem is that she is already asking herself, really. If that question had ever been answered in her own mind, or if she had already accepted and forgiven herself her errors in judgment, she would not be afraid to let you talk about it now.
Sometimes the people who hurt us are really running and screaming trying to avoid guilt and responsibility. I don't think we can make people take responsibility, but we don't have to pretend for them either.
The consequence she has to suffer beacuse she will not talk to you about this is your withdrawal. You should withdraw if you are unhappy with the pretense, but I'm guessing because she can be so "good" that guilt is a powerful weapon both she and you can use to keep you from creating and maintaining distance.
Distance may be the only thing that helps you come to terms with your own feelings.
You can try being realy frank with her one time: "I need to talk about this to be able to be comfortable in tour relationship." this way, you have given her an opportunity to meet your need, which is more than reasonable, and to take responsibillity and deal with your pain and hers too.
But offer this only once. If she does not give you what you need, then remember that refusal every time you start to feel guilty for withdrawing. That way you may be able to take small steps toward separating in a self-affirming rather than desperate sort of way.
I really em/sympathize.
T