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.
Am I a disgusting person?
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.