It's very important for your worldview to hold up that I be a stupid teenage girl. I don't know why you decided to see me as a different person when I hit puberty. I was always Davey. I was just your kid. ... It really sucks that you have no interest in the real me. You're going to believe whatever you want to believe about me and the truth is irrelevent.
You aren't really expecting an answer when you ask why I do something. You just want to get my attention so you can respond with an insult. "Here is why you are wrong; here is why you are stupid". There is no Correct Response to these types of questions. Why do I do the things that I do? Because I'm a bad person. Right? I used to think that there was a winning condition under which I'd be Enough for you, but that never existed.
I feel bad for you. You've been through a lot.