The big problem here is that you and your ex-boyfriend are not friends — and you’re trying to be — and it’s not working. 😕 A friend isn’t someone you sleep with. But even if you don’t agree with me on that, I know you’ll agree that a friend isn’t someone who’s there for you when he’s not dating, but is when he isn’t. That’s not friendship. That’s not loyalty. And that’s not where you should be investing your energy. 😉 The big problem is your relationship with your ex-boyfriend because he’s there when he’s single and tells you to stay out of his life when he’s dating someone. So, my advice is to move on and let him go. You had a relationship that ended. He’s not your friend. Next!
As for the micro-dynamics within that dysfunctional relationship, I would have counseled you to distance yourself from the drama. When your best friend confided in you that she was texting your ex, you learned something about her and let that relationship go. Then, your ex’s girlfriend started texting you to dump her relationship problems with your ex on you. Not a great choice on her part, but… I would have suggested you use a boundary with her because her attempt at drawing you into her relationship drama was inappropriate. I would also have told you not to show your ex the texts between his ex-girlfriend and your ex-best friend, etc. — because it was just stirring the pot (a pot you were not really part of). And now, your ex-boyfriend is asking you to get lost so he can try and work things out with his recent ex-girlfriend. Can you see, reading this, how there’s really nothing in this for you, except drama?
Move on. Let your ex-boyfriend go. Focus on healthy relationships — without gossip or drama. 🙂 You deserve that.