I over heard my mother talking to her friend about me, and she told her that I was probably just confused and 'thought' I was transgender because of the 5 years of sexual abuse I went through with my ex-step father. ._. I did my best to let her know I've known about myself being transgender since before she was even married to him, but I just don't know what else to do.
You could start by telling her there's no wiggle room; you are certain of who you are. I'd also ask her to assume you know yourself well enough for her to trust you about that much.
I would just tell her again very directly that this is who you are and it has nothing to do with your unfortunate experiences. Tell her she can believe it or not, but that's just the truth.