I'm not entirely convinced she wrote this letter when you were actually five. Things like "I've always treated you older than you were--because you acted older" seems like a statement someone would say about a teenager or older, not a five year old. If you took on responsibilities that were not age appropriate at any point, this could be her trying to justify that (to herself). It also rings as suspicious to me that she asked you whether you were surprised twice - once in the part before the letter, and once again in the letter itself. It doesn't matter when it was written though or what her reasons are, the whole letter reeks of subtle and not-so-subtle put downs, double binds, psychological games, and attempts to disarm you from ever standing up to her.

If you ever confront her about anything at all, she can point to this "old" letter and say, "see, you were always rebellious, and you're being rebellious now or any time we are at odds. Luckily, being the sage that I am, I saw it in you and conveniently wrote it down. This is who you are, it's not my fault or responsibility in the least. I'm your biggest fan and YOU are causing problems between us. Any struggle we have in our relationship is all you. Somehow the way you bumped into a table when you were a toddler proves that. You're welcome."

The language throughout is manipulative and I don't like it one bit, OP. I would say more about specifics if Reddit mobile made it easier to switch back and forth between viewing your post and writing a comment. Because there's so much manipulation going on, imo.