He has sent you a very clear message; one you're understandably not wanting to hear. He plans to pursue the other relationship, as destructive as that appears. Since he refuses to end the relationship, that leaves you in the position of accepting the fact he's seeing someone else or ending your relationship.
At a time in my career, I was employed as a marriage counselor. It sure sounded great, helping people get along better and be happy. Actually, it was the worst time of my professional life. I had couples that had literally exchanged gunfire. But that's another story. One option in this situation is to declare an open relationship. From reading your post, it's obvious that that's not an option for you. You sound like someone who's not willing to share. And, there's nothing wrong with that. You seem to have a high need for commitment and exclusivity is obviously a value for you. Therefore, it is obvious that you aren't getting those needs met in this relationship. Therefore, it's time to end the relationship as long as this situation exists. Obviously, if they break up then you might or might not be willing to resume the relationship.
I think you have to accept that you are just not going to get your need for commitment and to be his only girl met in this relationship. I think you need to tell him you won't be part of a harem and wish him well. Believe it or not, trying to hold onto the other person only drives them further away. Sometimes one person begins to feel "trapped" in a relationship. Those feelings often lead to affairs. Sometimes, when they see that the relationship is really in danger of ending, they decide they don't want out as much as they thought they did.
You need to link up with a support system. You're vulnerable right now, so that support system needs to be other women. Whatever you do, don't fall into the trap of having a "retaliatory affair." People who have affairs to "get even" always feel guilty and dirty afterwards. You'll know when it's time to move on.
You need to put some distance between you and him. Right now, you're emotions are probably overwhelming and confusing. You need some distance to get your head clear and start thinking rationally again. You need a close (female) friend you can trust to give you good advice during this time. You may need someone to keep you from running back to him when you feel lonely and desperate. Don't compromise who you are and what you believe just to "win" him back.