Real event OCD: should I just confess to get it over with?
Telling your partner, or the person involved, feels like the honest way out. Confession usually works as a compulsion here: relief for an evening, and a new person to seek reassurance from.

It is two in the morning, and the thought has arrived that would end this: just tell them. Tell your partner what you did, or might have done, at nineteen. Tell the person who was there. Confess, take whatever comes, and at least the secret will be gone and the investigating can stop. Honesty is a virtue. Surely the honest thing is the right thing.
I want to ask you to pause before you do, and to explain why, because confession in this theme usually does not do what it promises.
What confession is for
The reasoning goes: the doubt is unbearable because I am carrying it alone. If I share it, the weight halves, and the other person's reaction will tell me whether it was as bad as I fear. Either they will say it was nothing, and I can stop, or they will say it was serious, and at least I will know.
That reasoning wants an answer. It wants the question, was it as bad as I fear, settled by someone outside. And that is exactly what every other compulsion in this theme wants: the reconstruction, the rereading, the asking someone who was there what you were like. Confession is reassurance-seeking with the highest stakes available, and it works the way reassurance-seeking always works.
What actually happens
You confess. They react, usually with some version of it was nothing, or that was years ago, or I do not think that counts. The relief is enormous. For an evening, perhaps a few days, the weight is gone.
Then the doubt finds a gap. They said it was nothing, but did I tell them all of it? Did I describe it accurately, or did I soften it? Would they have said something different if they had really understood? And now there is a new question, and a new person to ask, and the investigating resumes with a fresh source of reassurance installed. You have added a participant to the loop.
Meanwhile, the confession itself may have consequences. A partner who did not need to carry this now does. A relationship has a new fact in it that cannot be removed. And none of it repaired anything, because there was nothing there that reassurance could repair.
When disclosure is right
I am not saying nothing should ever be disclosed. If there is something genuinely worth telling someone, because they have a right to know, or because a real repair is owed, then it should be told. But that is a decision to make with a clear head, in daylight, with a plan for what and to whom, and ideally having worked out first whether this is a conscience or a compulsion. It is not a decision to make at two in the morning as a way of stopping the loop for tonight. In CBT with exposure and response prevention, we can work out together what, if anything, is worth disclosing, and do it deliberately rather than as an act of relief.
The confession to the therapist
There is a version of this that happens in the room. People arrive wanting to tell me the whole account, in detail, so that I can render a verdict. I will not take the bait, and I will say so out loud rather than leaving you to interpret my silence, not because I am withholding but because telling you it was fine would last about an hour and would cost you a fortnight of progress. You will not be asked to give a full account of the event to prove you are unwell, and the treatment does not depend on my assessment of what happened.
What we do instead
What we are not going to do is establish what happened. The establishing is the compulsion, and it is the thing keeping you here. We map the cycle, the trigger, the memory, the meaning it has taken on, and every move you make afterwards to feel resolved, and we withdraw those moves gradually, by agreement. Confession is one of them. We let the memory sit there un-investigated until it goes back to being what it is.
If you are on the edge of telling someone tonight, there is more on how I treat real event and false memory OCD, and the decision can wait for daylight.