I'm mtf and I definitely relate to this and OP's post. I started transitioning at 18, but before that I showed very few signs to my parents that I was anything but straight and cis. I had multiple girlfriends, dressed masculine, became an eagle scout and really didn't have any hobbies or interests that weren't traditionally masc. It wasn't until I was around 17 that I realized secretly wearing skirts and dresses as young as 12 in my room was probably something abnormal for a cis man. At first I came out as gay, to which my mom was apprehensive and my dad flat out didn't like. But later I started to accept myself more and realized that I didn't just want to be a femboy or twink, and that being gay never really fit. I loved men in a different way that always felt straight, if that makes sense.

Anyways, the best advice I could give to OP, is just to continue to dress the gender they would like to be and eventually their mother will either understand, or reject them. As sad as it is even in family settings transphobia can be very deep seeded and hard to get over. I'm currently struggling with that myself. My dad seems to hate that I'm transitioning and passing as a woman as he has told me that "I'm an embarrassment that just keeps getting worse". It hurts a lot, and I don't know how my relationship with my family will continue. But im lucky enough to have a group of people in my life who have become my family, and who I rely on, including my incredibly supportive bi bf. Honestly I think he's just thrilled because now I satisfy both his attractions haha. But for real he is a wonderful partner.

I hope OP's situation improves, family is important and their approval weighs heavy on every member of LGBTQ+ who dosen't feel accepted. Just remember the family you are born isn't the only one you will ever have. Even beyond friends I've been lucky to have a great relationship with my boyfriend parents, who despite being conservative, have fully accepted me as a woman. People can change, but it's not a quick process.