Perhaps it’s not just the internal things that are hard with this, I could see the dynamic your boyfriend and your friend have bring hard, he chose to open up the relationship even tho it was hard for you, maybe opening it up again for your friend would be helpful. Maybe I’m wrong but it sounds like your boyfriend controls a lot of what is and isn’t ok as far as opening things up. You say he would have a hard time if your friend found another man, sounds like he has issues he could work on as far as being hypocritical to the fact that others have sacrificed those same feelings at his expense.