Don't let this ruin how you see yourself. Your therapist is right - you shouldn't feel badly about this. Kids do strange things. It's just too bad that it happened during an age where you were old enough to remember it now.
You'd be really surprised at how many people have done all kinds of things. You're not alone in that. I've done some "out there" things myself, back in the day.
Not only can we not judge any other people, we can't even really judge ourselves. Quite often, our criteria for judgment is flawed to begin with. So if that is the case, better to find a way to accept ourselves and the things we have done with the best grace we can, and move on.