I was just watching a video of an ungrateful young man that gets all upset, overreacts, insults them, walks away and starts drinking.
This might sound rude but honestly, I imagine slapping the hell out of that ungrateful brat at the very moment of getting blatantly disrespected. This would be my answer to that ungrateful brat:
''How dare you, I fed you, I was there when you were sick, reading your bedtime stories, taking you to school, providing you love, gave you an education and you're telling me I'm not your parent and a liar just because I don't share your blood, you realize many kids are dying and starving without any parent at all, how dare you talk to me in that manner''.