You gotta bail. Have some self respect here cmon...

Man, I've read too many doujins is what I was thinking while reading this.

You're still into her and your girlfriend is right to be honest. Such boundaries should be established and respected. Honestly, with the way you describe that Mia person, you're still infatuated over someone who shouldn't be your friend at all over what she did.

There's forgiveness and moving on and then there's this.