I forgot you Exist
This is said by someone who thinks they're the only one who matters, and they say it to cover up the fact that they're still mad at the other person and they probably got bullied by them.
'I forgot you exist?' You're still here, so why are you still mad?
You forgot I exist? I guess you're still bitter about that time you failed my math test.
I forgot you exist? You're still here, so you're not going anywhere.