Not every letter needs a recipient. Sometimes writing it is the whole point.
Even though you won't, writing as if someone will read it tends to pull out more honesty than writing "just for yourself" in the abstract.
Unlike a real conversation, this letter doesn't need to consider the other person's feelings. This is the one place you can be entirely on your own side.
If you keep circling back to the same sentence, that's usually the part that matters most. Don't edit it out for the sake of tidiness.
Some people keep unsent letters. Some delete them right after writing. Both are fine — the writing did its job either way.
Write it anonymously if you want — no one has to know it's yours.
Write Your Unsent Letter