I'm lost at how to get along comfortably like this?

I'm a FtM kid in high school, and I'm not sure where this should go so I'm gonna say it here, I hope it's not wrong I get so embarrassed by doing things wrong, anyways, I'm out to my family as both "gay" because they couldn't understand when I tried explaining Demisexual Panromantic, and I'm out as trans*. And to a few friends, including my precious girlfriend down in Texas. My mom says my preferred name can be my nickname, but the only things I can change is my clothing and hair. She doesn't think I can go and change in the right locker room for gym, use the boys bathroom, and all that because whether I feel male or not, legally I'm still a girl and my name is still my girly-as-hell birth name. I've heard of people using the boy's locker room and boy's bathroom in school when they are trans*, and being called by their preferred gender and all, so I don't understand. My family is a very supporting family, in most matters, tho they are absolutely horrendous at treating people properly, being polite, and not making their kids think they don't matter and are shitty and worthless on accident. I used my christmas money to buy a binder, and I have a boy's hair cut as well, I sure look the part. I'm just so lost. I get so anxious and sick and uncomfortable during gym and usually stand outside the locker rooms for a while to wait for all the girls to change and go out to the gym because it just feels so wrong to change around them, and even if I'm about to piss my pants I absolutely never use the bathroom in school because I just don't know where to go. I'm really shy and soft-spoken, even to my friends, and so really the only people who even call me by my *preferred* name are those close friends who I know on the internet and my mom. I just don't know how to do any of this, I'm only a freshman and I've only recently started to come to terms with how I feel and all that, though I've felt this way for as long as I can remember. I'm just really lost at how to get along comfortably like this, and if someone could help me that'd be great.