I agree with BohenianMLHR81. It sounds like D is not being honest with himself or you, for whatever reason. He sounds like he wants a mono relationship with you, but something is preventing him from admitting it. Actions speak louder than words. His actions say he wants you to himself. My suggestion is to keep telling him this. See what he says. If he can't be honest, then real communication with him is not possible. If the drama continues, then my suggestion is to be honest with yourself and decide for yourself what is best for you.
My own life experience has taught me that if someone can't be honest with me, then I have to disregard what they are saying and make any necessary decisions without considering them. This has worked well for me regardless of whether the intention behind the dishonesty came from wanting to be hurtful or because of low self esteem. Both reasons are still dishonest, and therefore ultimately destructive.
My best guess is that D knows you are closer to C in a lot of ways that D wants to be with you. D feels he has already lost a big piece of his relationship with you. I think he knows if he insists on going back to monogamy with you he will not have the same relationship he once had anyway. He knows there is a chance you may leave him entirely for C. The middle ground is to pretend to be OK with it. He is learning pretending doesn't work.
The truth is you are not responsible for D's feelings. He is. It is also true that if you eventually decide that leaving D is the best thing for you, any feelings you have about that are your own. Keep that boundary front and center. It will help you honor you.