Why don't some people not wanna drop the subject?

My friend thinks she is trying to help when I did a dumbass thing. I stupidly let my cousins ex girlfriend in the house but I was in a rush to college. I wasn't even thinking straight. My friend was like, "I heard what happened. What were you thin.." I cut her off saying, "I don't wanna talk about it. I feel bad and dumb enough as it is so legs just drop it!" I ended up telling her anyways. She was like, "You should know better. Never let people in Your house like that. Your dad is going to lecture you and tell you off." My brother was like, " (lets call her T) T, get out of here! You're not helping! Dont make her feel any worse."
She said she was just trying to help but she wasn't. I sternly told her I don't need a lecture.
We all screw up sometimes and she expects everyone to be perfect. She kept bringing it up and wanting to help but I bluntly told her NO and quit bringing it up and her made me feel worse.