I forgot you Exist
This phrase is used by people who are so shallow they think the world revolves around them, and they're just trying to cover up the fact that they're still mad at the other person.
'I forgot you exist?' Oh, you're still alive? That’s a shock.
I forgot you exist? I guess you're not going to let that go.
You forgot I exist? You're still mad I didn't ask you to my birthday party.