Dear hanidashhan:
As to your question: “What should I do to fix everything?”
You can’t fix everything and it is not your job to fix everything…because you didn’t break everything.
Your ex boyfriend chose to end his relationship with you, start a relationship with another woman, then continue an intimate relationship with you. These are his choices. Your previous best friend chose to contact his girlfriend and tell her about the going ons, this is her choice.
Your choice is to be intimate with your ex boyfriend while he had a new girlfriend oversees. You are only responsible for your choice. You are only responsible for your behavior, not for the others.
So tell the boyfriend that it was a mistake for the two of you to be intimate when he had another girlfriend (notice a mistake on the part of the TWO of you). If you know his girlfriend, you can apologize to her about your part in the intimacy (don’t apologize for his part, you didn’t force him). And regarding your former best friend, I don’t think she is your best friend anymore. She should have told you her feelings about you and the ex, not the girlfriend. She should have told you that she is thinking of telling the girlfriend before telling her.
anita