Faye, it might be really painful for you but I think at this point you have to keep your dignity and walk out of the relationship. If you don't do it now, you will have more misery and struggle and your relationship will end eventually anyway.
Why he needs you in his life? The answer is very simple. He feels very important when two women are in love with him, fighting over him, being jealous of each other. Without one of you two it wouldn't be the same.
It gives him high of adventure, of something risky in his life.
But there is nothing about You in his feelings. It' about his Narcissistic self.
Plus, it might be very serious relationship for you, but obviously not for him. If he was afraid to loose you he wouldn't hurt you that way.
Agreeing on that you are losing more and more value in his eyes. Instead of appreciation for understanding and patience he will look at you as at something with no value, no respect for yourself and no independence.
If you want to win in the situation you have to give him a choice - either it's only two of you or you are out.
If he doesnt' agree then he doesn't love you and need you enough and it wouldn't work out any way.