Help with people believing me and help telling my friends

Okay so this post If about a couple things I need help on. The first is having people actually believe me when I come out as trans. I've so far told my mom, my boyfriend and my best friend. My friend has been extremely supportive and she was actually the first person I told since she herself is trans. I told my mom not long after expecting the support I got from her when I came out as bisexual. She was initially great but hasn't been since then and it's been a few months. Her first response was a little bit disappointed I mean her only daughter telling her that they are a girl it's to be expected. Now after that she told her fiancé which is fine and I expected it but then she told my little brother as though it was a joke. My twin found out by getting on my moms phone one night and reading the texts between my mom and I. Now my moms fiancé has been fine he doesn't really say anything about it but my mom and both my brothers have been making me feel horrible they keep telling me to grow my hair out and wear makeup and look like a girl. It's been horrible especially when my mom has been pestering me about wearing a dress to her wedding. Now my boyfriend has been similar to my mom. He was extremely supportive when I told him but hasn't brought it up since and he acts extremely disappointed when I get my hair cut because I'm not growing it out and he has been saying he won't take me to prom if I don't wear a dress. (I don't think we will make it another month together though) I am now extremely worried though to tell my dad and stepmom because I'm afraid they won't believe me either and I know some of this is my fault because several months ago I told my parents I was trans and then they were all really nice just a bit sad that their only girl was actually a boy and I ended up telling them a few days later I thought I was wrong and that maybe I'm just a tomboy. Now that I've spent about 6 more months looking more into everything I have figured that I am actually trans and am feeling ready to come out at school which means it's time to tell my dad.
I also wanted some advice for telling my friends. My best friend isn't much help because she told her closest friends before coming out at school but at the time it was just my boyfriend and I that she was friends with now our groups much bigger. I'm not at all worried about my friends being accepting I just don't know the right time to do it. I have a couple friends I don't get much one on one time with which makes it hard because that leaves lunch which would be a great time to tell all of them except it would be too easy for someone to over hear what we are talking about. I think my friends should be told before I come out fully at school I just am not sure when to do it or how to actually get my words right because I stumble over my words when I'm not nervous.
Thank you for helping me.

 

A friend of mine is going to help me talk with a counselor at school that helped her come out so she may be able to help I've just been dealing with my mom for months now and I'm so frustrated with it.