You're alone because you haven't stopped this nonsense.
It's time for you to give her an ultimatum: You or him. And make sure she knows that if she chooses him, that you will need to end the friendship in order to give your heart time to heal and to move on. Not to mention that it would be entirely inappropriate for you to date others with all this behind-the-scenes talk going on with this girl in the background.
Yes, she will get upset. But enough is enough.
Never regret anything that has happened in your life. It cannot be changed, forgotten or undone. So, take it as a lesson learned and move on.