I'll be honest, it sounds like you told her the name more to "push her buttons" than anything else. You knew she wouldn't like it and react poorly. If you really wanted to keep it a secret and avoid negative opinions, you would have kept your mouth shut.
I find it a little hilarious that you say you are keeping the name a "secret from everyone" when you've already told a bunch of people. What's the point?
You post comes off like you've finally gained the upper hand in some sort of twisted power struggle between you and your mom. If that makes you happy, congratulations.
I told her she wouldn't like the name and in hindsight, I would have waited, but she is the type who would have told me in the hospital after the baby is born that she hates the name and I didn't want anything giving me a bad memory of that day, so I was hoping she would get used to it by the time the baby got here. I could not even fathom that she would suggest using the middle name instead.
And we have not told a "bunch of people" - a friend knew in advance because she mentioned, when she was pregnant, that she liked the name for a girl and I asked her how set she was on using it (it wasn't even in her Top 5, she just liked it) and I told her we were hoping to use it. She was already leaning toward a different name anyway (and ended up having a boy) so it was a non-issue and she kept it a secret for me. I also told my best friend because she's my best friend - both of these women have known for years and never told anyone.
Other than that, my parents are the only ones who know (my in-laws opted not to find out because they want to be surprised). My sisters don't know, my circle of friends don't know, the people who have asked in my church and on Facebook don't know - so, yes, it is a secret. I don't mind mentioning it here because the odds of a non-pregnant acquaintance coming to a baby website, stumbling across the message board, and finding this post and connecting it with me are next to nothing.
My mom is over-critical and we fight occasionally, but this situation was enough that I wanted to vent about it because it was extreme for her.
As for the "twisted power struggle" - I have no words.