For the letter you keep not sending.
Write it to someone who can't answer back. Someone who died. Someone you lost touch with. Someone you're never going to say it to out loud.
Say the whole thing, in one go, the way you'd never manage in a conversation. Then seal it.
Sealed means: it leaves this page and isn't stored anywhere. No account, no inbox, no server holding it. Nobody reads it. Not the person it's for, not me.
It's gone from this page and I never had it. Nothing here remembers what you wrote.
If you want to keep the letter yourself, take a copy now. Fold it, put it in a drawer, leave it somewhere it stays. Or don't.