I hope you don't mind but I've made some changes to the format, a big wall of text can be disconcerting and put someone off reading a letter.
I've also made a few wording changes and underlined some things you may want to rewrite later, and please know that what I say next is objective and not meant as a criticism of you or a chastisement, while aggression can be considered a tool in a discussion it can also close your audience off from the ideas you are trying to convey.
I said I'd do this this way cos I'm not very good with
words and this will allow me to explain everything...
When I came out to you back in March, it wasn't some sort of joke, this isn't some sort of phase I'm going through, I've finally found a way to describe how I have felt my whole life!
If you think back you'll see it explains a lot about me in my life. I have always wanted to be treated as one of the boys, I can even recall saying that I wished I was born a boy a few times, when playing mammy and daddy type games I was always the daddy or I wasn't playing, at Christmas I was kinda jealous over the boys getting male things and I not and eventually you started buying me action men, playing games in school I was nearly always part of the boys group, I'm sure there are many more examples that you can probably think of which I can't, but nevertheless all the signs were there I'm sure you probably noticed them.
So for you to react the way you did is disappointing, you thought you could just brush it off as if it were nothing, not taking my feelings in to consideration at all, all you care about is what society thinks, why bother having children if you're not prepared to accept them for who they are, I'm not some sort of doll for you to toy around with I am a human being and I have my own life to live and my own happiness to fulfil.
Cos I know what you're going to ask "can you not just do this for me", and the answer is no, I've spent my whole life pleasing you, I'm not a 13 year old child anymore that you can threaten with this and that like you did when you made my life hell for years after you found out about my sexuality.
I know what you were thinking, if you didn't mention it or discuss it with me that it would just go away, well it's not going to, I'm a grown person and I want to be happy and this is going to happen with or without you. I would love to have your support, I realize this is hard and you have said you're uncomfortable with it and out of respect I have not mentioned it.
I've slowly tried to exptess myself subtly: ditching my handbag and getting a wallet, cutting my hair up (which I know I did a few weeks before I told you but still), buying men's clothes (which I had always done, but you seem more against it since I came out to you...), men's deodorant (which was in my room for 4 months, then I go to buy my 3rd bottle and you happen to be with me and just stand there with your mouth wide open staring at me and question why I bought out of the men's section and then call me stupid).
I know reason you're uncomfortable is because you don't understand, I want to help you understand please give me the chance. I am the exact same person I have always been, I have always identified as male more so than female and you know that deep down which is why I was shocked by your reaction back in March, like you could have just said that you always kinda knew or something but instead you tell me you've discussed it with your lesbian friend and basically try to tell me that I'm confused and you don't know what to do for me.
All I ask is you think through all this, take as much time as you need, I know it'll take time to take it all in and if you choose to support me it'll take time getting used to the whole adjustment but I'm the exact same person I have always been, I just want to be me.
I hope your mother comes to accept your transition.
Hugs,
Sarah