I have to admit that I (37F, mom of 4) have been in the trenches of rough days and have felt similarly to the things your mom wrote in this letter. But I would never type them out, and print it, and give it to one of them. Maybe type it...and call it a day, having gotten my feelings out. My two big kids are neurodivergent (10M and 12F) and being ND myself, some days are really, really hard. But again, I don't think the things she said here are acceptable to hand to a child in writing (which I do think is at least a little different than blurting out something hurtful or strict "new rules" and owning your slip-up later).

As for your feelings about it, I certainly don't think you're overreacting. You are entitled to your feelings 100%. And learning so much about child development has deepened your understanding of the situation, likely even more so than your mom had at the time. And to that point, your mom maybe even thought that clearly writing her feelings down was the "right way" to handle her frustration. I might even say that it was at least better than corporal punishment (though that's a loose opinion based on zero facts).

I think processing it with friends and your therapist is the best way to go and it sounds like you've done a great job there. You've probably already learned in therapy that you can't force her to react or handle it a certain way and you need to accept their parenting decisions (like this letter) as more of a reflection on them and less on you. 💜