First off, you don't need to apologise for posting or for how much you write. Secondly, I'm not surprised you found that triggering. Is there definitely no way you can get out of it? If I remember rightly your T is part of your school, any way you could discuss this together? And/or does this teacher seem like someone you could maybe approach outside of class to discuss these issues?

If you're stuck in the class, I find myself wondering if it might help to focus just on music when you consider these things. So, for example, you could say: "If it weren't too late, I'd learn to play such-and-such instrument."

With the letter from your 80-year-old self, I suppose the thing to remember is that you're making it up, so you don't have to think of it as being about you - if it helps you, you could invent a character and write a letter from their 80-year-old self instead of yours. Again, maybe keep it on the subject of music (though I don't get what writing this kind of piece has to do with music, but anyway). Maybe you could invent an outlandish story about someone who runs away to play violin in a circus and then goes to prison for stealing a piano.

That's kind of a silly example, but my point is: if you just need to get through the class any way you can, maybe the answer is just to make things up. You could create a character in your head and write from their point of view, not yours. When you approach these questions, think about them, not you. It's not ideal, as you won't work on finding your own inner musical voice, but frankly I don't see how any of this is going to help you do that anyway...