> It was puberty that did it.

Nice starting line, it gave me a good indication of what the story would be about, the frame of mind of the narrator, and served as a firm but gentle hook.

> Suddenly there were all these expectations for how I should look, what I should wear, and how I should behave.

I feel like this might be stronger if you refer to specific to the expectations that were already there, but contrast with how they're now tighter. For example, pants getting replaced with dresses, the wild hair needing to be combed, details like that.

> So I learnt to play the part I'd been cast in. Someone who wasn't me. But at least she was happy — or good at pretending to be.

Really great line here!

> And

There's five or so paragraphs that begin with "and." I think it works better sparingly but this much feels repetitive. See if you can just chop off the and in a few places or switch up the word order.

> And the first person who needs to accept me, is me.

Having said that, this "and" I do love, though. Very level headed and grounded protagonist!

> It arrived today, waiting on the porch when I got home from school.

Since the story is a teenager writing in their journal, you might try leaning harder into the journal-ness and kicking the second half off with a sentence about how they're relieved, or their hands are shaking, or some other kind of line that a person would write when they've rushed back to their room to write something down.

> Not all of it. Not all the half-thought thoughts and questions and worries and secrets. But I told them enough.

Again really nice sentence that feels real and touching, especially the half-thought thoughts. I love how varied your sentence length is throughout, coupled with the repetition, it makes the reading more exciting and also feels true to how a teen would write.

> , and of that, I am proud.

I'm really on the fence here, I've tried taking this off and reading the last sentence without it, but I'm not sure if the ending works better with or without it. You might give it a shot too and see.

Thanks for the lovely, sad but heartwarming story Rainbow!