I don’t think you should beat yourself up over this. As you stated, you wouldn’t ever do anything to your nephew, and you wouldn’t let anyone else do anything to your nephew. When you start exploring sexuality, there’s lots of things you don’t know. Some of these things people don’t know is what is okay and what is not okay to do. You obviously know now not to do what you did when you were a kid. You’re not a danger to children in any way, you were just a dumb kid doing dumb things, like we all were at one point to some extent. It’s actually pretty common for people to feel heavy guilt in adulthood over childhood sexual exploration. People do dumb and inappropriate shit when they’re kids. As long as you know not to do the same dumb shit now, don’t feel like you have to live with the extreme guilt you feel over this. You are not a monster.