What you Need to Know about Love"
That's from a Not Just Friends, a book by Shirley Glass that discusses the need for boundaries in relationships and the slide into affairs.
Your girlfriend's friend never had any interest in being a friend to your relationship. She made it clear from the very first meeting that she wanted you for herself. You are human and responded. It's time to put on the brakes to your slide into cheating and think about what you really want. I doubt that you want to be the guy who cheated on his girlfriend with her best friend. So don't be him. Withdraw from the friendship and focus on your relationship with your girl friend.