I agree with the other's responses here. BUT, let me add, the same way that you are nervous to talk to them about it, they have the same nervousness to mention it to you.
Take a breath and tell her you feel that way. If you want to be non-committal you could say something like, "Mom, I would like to see a therapist about ..." from there you can go with all sorts of things like:
1) my uncertainty of being a boy/girl
2) some stuff
3) some stuff specifically gender identity
4) possibly being transgender
5) ...
Here's a question - is it possible you were born Intersex and/or a "decision" was made.
You could approach your mom from that direction. "Mom, is there something about my you know but are not telling me?"
You are probably too old to ask the way I did. When I was 4 I walked up to my mom and asked "Mom, was there a mistake made? Am I supposed to be a girl?" (I remember this like it was this morning!) That was in 1966 or 1967 so the answer was... "Ha Ha...." (the rest didn't matter) and I buried the question for 50+ years.