My boyfriend is dating me and another girl
Don't ever talk about your broken relationship or your hopes for a future with him.
Either way this borderline passive aggressive act is done with purpose although you will find most men don't even realize they're doing it.
If that guy has too many insecurities and is constantly searching for validation he will (at times) feel helpless.
As always, when a man feels "helpless" you're likely to see parts of him you may not enjoy... Now I don't want you to believe he's not strong enough for you or that you should dump a man who's not secure enough for you.
Your guy chose her over you and it's really as simple as that. That sounds heartless but you're not going to do one thing that will be insulting to her.
You're simply going to befriend her and him and show that you want to be a supportive friend to both of them.
If you can overcome the pain you are feeling over the break up enough to take a position in his life as a friend, you'll have a chance to lay the foundation for more in the future.