I regretfully outed a girl I barely knew by telling a friend and it still haunts me today
Why do you think it haunts you?
Because I know it wasn't the right thing to do
What's done is done. You can't change the past. It's best to learn from your mistakes and move on.
I don't think that it was right, but it sure felt good to stick it to her when I outed her.
You are bigger than your mistakes and you shouldn't keep hanging onto it all. You can use this experience to make you stronger and prevent such things from happening again. Let go of those vengeful feelings.