I would tell my pre-transition self: (my self during my denial period from age 18 to age 26 or so)
1. It's not too late. Hormones WILL work on you.
2. Get going on it. You think that you're over your trans desires, and that everything is going to be fine in your life. It's not. You're living in denial. You're miserable and unmotivated, and you know that you're miserable and unmotivated. And even when you have everything you thought you wanted, this feeling will not go away. Healing is a long process, but it can't begin until you finally admit to yourself that you really do want to be female. Every day that you wait is just another day of hating your body that you'll have to put up with.
3. You do have the courage to go public with this. You just don't know it yet. Don't be ashamed of who you are.
4. Everyone that you love will be completely fine with it. Don't worry about their expectations. They don't love this perfect self that you try to be for them. They love YOU, the real you. Your mother is hurting because she saw how depressed you were in high school and she felt powerless to help. When you finally admit why to her, she'll cry and say "I've finally got you back," and you'll be closer to her than ever.
(If we're going to go back to being even younger, so that I can talk to my middle school self:)
5. Come out as trans NOW!!!! Stop your voice from changing, and stop puberty before it happens. You know that you're absolutely petrified of losing that beautiful voice, and you know that you're jealous of girls in every way, and you hate body hair and hate your offending lower male anatomy. It's not because you're a weirdo. It's because you're transsexual. You really are. And if you don't do something about it now, you're going to spend the entire rest of your life wishing that you could have. You will miss that voice FOREVER. And if you do it now, you'll look so much like a genetic girl that it will be impossible for anyone to tell. Hormones, damn it!!! You're not doomed to suffer through a male puberty that you hate every single second of. If only you had known about testosterone-blockers...
6. Quit caring so damned much about what other people think about you. It's not your problem, it's theirs. They're projecting their own social insecurities onto you. Don't let them get to you. Live your life, and be proud of who you are. Don't deny yourself for the sake of fitting in once you get to high school. It is the single worst mistake that you will ever make in your entire life. Fitting in won't make you happier. You will only change from suffering publicly, to suffering in silence, feeling like not a single person in the world knows the real you.
7. You know how you wish you could be a girl? Well guess what, kiddo? You are going to be one some day. And it really is going to feel every bit as wonderful as you imagine in your little nighttime fantasy stories that you write. Just wait until you feel that amazing feeling of someone telling you that you're sweet and that you're beautiful for the first time. It will blow your freaking mind, and you will NEVER feel so happy.