Unfortunately this is more often the result from disclosure. I can only think that it's because if she accepts this happened to you that it reflects upon her as not being a good mother that protected you. It might not be a conscious effort, on her part though.

Don't worry about your memories being the way they are. The brain just doesn't recall things just as they are, and through dreams they are refreshed but can become distorted somewhat. That isn't to say that what happened to you didn't, it has little to do with the actual, total truth.

I'm glad you have your T.
Maybe you can keep the communication channels open with your mom, and educate her as you also learn how to deal with this trauma.