It sounds like a school exercise. It is one of the few pieces of writing that reliably changes something, and the reason is unglamorous: you are a poor witness to your own life while you are inside it, and a letter is a way of leaving evidence for someone who will read it with fresh eyes.
Why memory is not enough
Human memory does not store the past and replay it. It rebuilds it each time, using today as the reference. If things are good now, last year gets quietly improved. If things are hard now, last year gets darkened to match. This happens beneath awareness and it is very difficult to argue with.
The practical result is that people consistently underestimate how much has changed, because the version they are comparing against has been updated without their permission. A letter is the only copy that does not update.
Memory edits toward the present. A letter does not.
What to put in it, and what to leave out
Hopes age badly. A letter that is mostly wishes will read as either smug or sad in twelve months, and either way it tells you very little.
What ages well is detail. What you are worried about this week. What a normal Tuesday looks like. Who you saw. What you are avoiding. What you keep meaning to do. The specific and slightly boring material is what makes the letter useful, because that is the layer that will have changed without your noticing.
The honest version
Write it as though nobody else will read it, because nobody else will. The temptation is to write to a future self who is doing well, which produces a performance rather than a record.
Include the thing you would not say out loud. That sentence is usually the one that means something when it is opened, and it is often the one that turns out to have been the centre of the year.
Sealing and forgetting
Put a date on the outside. Put it somewhere you will not casually see it. Then genuinely forget about it, which is easier than it sounds, and is most of the mechanism.
What happens on opening is not usually nostalgia. It is a small shock at how much of what dominated a year turned out not to matter, and a corresponding clarity about what actually did.