So yesterday I came out to my mom! Not just about being trans, I told her I was queer too. She was amazingly cool about everything, but I'm pretty sure that's because she knew already.
I told her I was attracted to men and women (and people outside of the ~binary, too, but I figured that might be too much to explain). She then asked me if I "wanted to be a man" and I was like "um, maybe?" This exchange followed:
Mom: (confused but jokingly): So you're a bisexual transgender??
Me: Yeah, I wanted all the letters.

I feel pretty good about everything. \o/ She wants to get together to talk more because she has questions about the whole trans thing, I know she doesn't quite get it. But overall it definitely went much better than I could have expected.

edit: I also told her I want her to pick a new male name for me. She was like, "it's hard picking a new name", so idk if she quite gets how much it means to me. But I really want my name to be somethings she chooses, so hopefully I'll manage to press on her the importance of this.