Don’t allow her to change the subject when you try to talk about your gender identity. Whether she realizes or not, by doing so she is stripping away your identity in order for you to conform to what she wants or desires you to be.
You have to talk to her without allowing the subject to change. Your gender identity is important and valid, and shouldn’t be brushed aside by anyone, even a significant other. It also isn’t healthy for you to be in a relationship where your girlfriend does not take your gender identity and your comfort with yourself in that identity seriously.