When I was 13 years old I did something very wrong. Back in the day in my middle school was fucked up trend: slaping girl's asses. I didn't have the courage to do it myself so when we had family reunion I decided to practice with my 5 years old cousin. When I was playing with her I slightly spanked her butt over the clothes multiple times. Then I continued with slightly pulling her pants down and then I slightly spanked. Once I had pulled so much that I saw her private parts and in that moment I had realized that I messed up. Since then I have never repeated such disgusting behaviour, yet my guilt and selfhate for this action has been slowly increasing.
Nowdays with my cousin I have good relationship, I sometimes help her out with school work. She doesn't seem to be uncomfortable around me.
You did something due to peer pressure at an age when kids are obsessed with fitting in. In the course of doing so, you realized it was wrong and you stopped. Even now, presumably years later, you still feel bad about it. I'd say you're good, every single one of us has done things (often involving inappropriate actions) we look back on and regret. You're fine.
Exactly this. All people do things that can be considered wrong. The difference between good and bad people is the good ones realize it, and change their behavior. This is what it is to be human.
I hate to break this to you, but kids are idiots. Their brains are still baking, so they do dumb shit all the time.
There's a reason that kids have a separate prong of the justice system in many places. Everybody knows the sheer stupidity that can happen because they, too, have done abysmally stupid shit.
Now, this one was pretty fucking bad. But it's still just an idiot kid being an idiot, not some predictor of the adult the kid grows into remaining an idiot.
Mind you, plenty of adults are idiots too, but the ones that don't repeat a given form of stupidity are ahead of the rest. So you're fine. Let go of the guilt, stop beating yourself up, and move on.
You clearly knew what you were doing was wrong, you've thought about it since and I'm presuming that you haven't done it since. Making a mistake and not making it again is the right thing. So no, not a disgusting person.
Oh fuck no. Don't apologize to the girl or you WILL have something even bigger to feel guilty about. How much random shit do you remember when you were five years old? I guarantee she doesn't remember a bit of it and never will, but you could easily plant false traumatic memories in her brain. That's just fucked up every way you look at it.
I didn't know that I have good advice for you - whether you should talk about it or just put it behind you - but be aware that 13-year-olds do some pretty awful things, just generally. It's that perfect intersection of dumb kid and new hormones. What you did at 13 doesn't define who you are today.