Looking back I couldn't stand her... I couldn't wait to get away from her. I am pissed at myself for allowing myself to get used by this parasite!
If you couldn't stand her, this probably wouldn't be so hard.
Intellectually, you know this was an adjunct relationship for both of you. There are a lot of alluring things about it - the push and pull challenge - the reward - etc. The play. The game.
You're caught up in it. Understanding why will help you get to a better place.
What is it about the game that pulls you in? What is missing in your life that is fulfilled here? How has this affected your relationship with your girlfriend?