I'd like to share a little something I've done lately to help deal with people calling me "he," "man," etc. Who knows, this same technique could help you out. These male gender labels are just words, and they aren't even meant offensively. I know that. Still, they can get under my skin because it reminds me of how I'm not seen for who I really am, and that leads to more self doubt and shit like "why don't I just be honest with everyone I meet," and "is there even a point in being honest with everyone I meet?" Lately I've developed a simple technique for getting over those stupid obnoxious thoughts. Whenever I catch myself having them I just fill my head with a big WHAT.EVER. I've been over this way too many times before to keep wasting my mind on it. I say WHATEVER in my head and focus on what's in front of me. I'm feeling so done with getting upset so easily. I want to save my impatience and frustration for when bigger problems in life come along that actually warrant my unhappiness. I want to move on from letting little things chip away at me.
I've started considering my public persona (fully female) to be similar to one of my gaming characters. My personality informs how I portray that character, but ultimately it isn't me.
Well, if that works for you, it works for you. Personally, I would recommend instead of "whatever" every time someone does it, remind yourself "I'm a girl" or "the word is she" something like that. That's what I do. Then again, I'm fully transgender, and your a demigirl, so that might be a little different, but still that's what I do and that is my advice.