Some apologies are years overdue. There's a person you hurt a long time ago, and the memory still sits with you, and lately you've been thinking you should finally say something. Reach out. Own it. Try to make it right. The impulse is good. But an apology this old needs more care than a fresh one, because the longer the gap, the more it matters to be honest about who the apology is really for.
Start there, before you write a word. Are you reaching out for them, or for you? An apology this late can be a genuine gift, and it can also be a way of handing someone your guilt to carry so you can finally set it down. If they've spent years healing and building a life past what you did, an unexpected message dragging it back up can reopen a wound they'd closed, all so you can feel lighter. Sometimes the most respectful thing is to sit with your own remorse and not make it their problem again. The test is simple and uncomfortable: are you prepared for them to be unmoved, to not reply, even to be angry? If you need the apology to go a certain way to feel okay, it's still about you, and it isn't ready.
If you've sat with that and you still believe reaching out serves them, or at least won't cost them, then write something that asks for nothing.
Own the specific thing, plainly. An apology this old carries a particular pull toward fog: "if I ever hurt you," "however you remember it," softening the wrong into something vague enough to be comfortable. Don't. Name what you did. Vagueness reads as a person still protecting themselves, and the whole point is that you've stopped.
Something like:
I've thought about this for a long time and I want to say it, with no expectation of anything back. What I did when I told everyone your secret was a real betrayal, and I knew it then. You trusted me and I hurt you for no good reason. I'm sorry. You don't owe me a reply or forgiveness. I just needed you to know that I understand what I did, and that I've carried it.
Notice what it doesn't do. It doesn't explain itself or list the ways you've grown since, and it doesn't ask to be let off the hook. It names the wrong and releases them from owing you anything. That last part is what separates a real amends from a request for absolution dressed up as one.
Then expect nothing, and mean it. They may not reply. They may reply with anger you have to receive without defending yourself, because they're entitled to it. None of those outcomes makes the apology a failure. You didn't send it to get a result. You sent it because it was owed, and an owed thing is worth saying even when it buys you nothing, maybe especially then.
That's the whole shape of it. Be honest about your motive and name the wrong without softening it. Then let go of the outcome completely. An apology that asks for nothing is the only kind, this late, that means anything at all.