Feel like I'm the worst daughter...

Long post warning...

So little back story... I was 11 years old when my life changed forever.. so I thought.. when I was 11 I found out the guy I would call "dad" was not in fact, my real dad.. I was lied to.. so after a few months of searching I finally found out who my dad was.. he was the total opposite of everyone on my mom's side.. he didn't cuss.. he didn't drink.. he didn't do drugs.. he didn't smoke.. nothing. I moved in with him the day I found out he was my dad (spent quite some time together before the results came back) and that day I became the biggest "daddy's girl" we were inseperable. When I turned about 20 moved states to start new. I believe it was that, that sent my dad over the edge.. he ended up getting really badly addicted to pills.. when his Dr found out he was abusing the medication, they stopped prescribing it to him.. so he then turned to alcohol. He drank so much each day it had destroyed his liver.. when that happened the liver let out a lot of toxins and it ended up getting to my dad's brain and he now does not remember anything.. he doesn't remember raising me, he doesn't remember my son, he knows nothing about me or my life anymore.. the man I talk to.. the man I used to call dad no longer sounds the same. And it breaks my heart more and more every time I talk to him. And as horrible as it sounds.. I have started stepping back.. it hurts so much that even tho his body is here, he himself is gone.. I've stopped responding to every text and every phone call.. in a way, I feel as if I have accepted the fact that he's gone and making it to where when he really leaves it won't hurt so bad? It's like I'm grieving when he's still alive.. I have no idea what to do.. it's hard with every conversation having to explain who my son is how old he is.. telling him that I'm married and then 5 seconds later answering the same questions cuz he doesn't remember.. any advice on what to do?! I love my dad and I feel like such a shitty daughter...

If you read all of this... Thank you..